MUDD kicks off the week…
A very gentle and enjoyable solve for a Monday morning.
Thanks MUDD!

ACROSS
1. Hobnail safe when hammered in (11)
FASHIONABLE
(HOBNAIL SAFE)* (*when hammered)
7. Cut identified (3)
SAW
9. Gather some moss, a majority rolled over (5)
AMASS
([mo]SS A MA[jority] (some))< (<rolled over)
10. Predatory bird after grub, as flying — something sweet in that (5,4)
SUGAR BOWL
OWL (predatory bird) after (GRUB AS)* (*flying)
11. By grave, keep going (9)
PERSEVERE
PER (by) + SEVERE (grave)
12. Nothing beyond infernal world — what do we have here then? (5)
HELLO
O (nothing) beyond HELL (infernal world)
13. Exclusion by management when hair revealed (7)
LOCKOUT
LOCK (hair) + OUT (revealed)
15. Kill noise, hullabaloo finally muffled (2,2)
DO IN
DIN (noise), [hullabalo]O (finally) muffled
18. Considered material (4)
FELT
20. This vessel’s small and powerful, but has got a failing (7)
TUGBOAT
(BUT has GOT A)* (*failing)
23. Theory in truth unchallenged (5)
HUNCH
[trut]H UNCH[allenged] (in)
24. Something afoot, seemingly — as private person mentioned? (5,4)
INNER SOLE
INNER (private) + “soul” = SOLE (person, “mentioned”)
26. Waspish given sack — hostile exchange (9)
CROSSFIRE
CROSS (waspish) given FIRE (sack)
27. Panel twiddling thumbs when in conference? (5)
BOARD
“bored” = BOARD (twiddling thumbs, “when in conference”)
28. Right to an opinion, for example (3)
SAY
29. US river, wet and grey (11)
YELLOWSTONE
YELLOW (wet) and STONE (grey)
DOWN
1. Mark European post with colours? (8)
FLAGPOLE
FLAG (mark) + POLE (European)
2. Plant put on pebble (8)
SHAMROCK
SHAM (put on) + ROCK (pebble)
3. Matter found in napkin, perhaps, when surface cleared (5)
ISSUE
[t]ISSUE (napkin, perhaps, when surface cleared)
4. Most curious where one sits when relocated (7)
NOSIEST
(ONE SITS)* (*when relocated)
5. Offer impressing leader in gaslighter, that man a narcissist (7)
BIGHEAD
BID (offer) impressing (G[aslighter] (leader in) + HE (that man) + A)
6. Funny girl then, a human (9)
EARTHLING
7. Adagio, where bass appears in secret (6)
SLOWLY
LOW (bass) appears in SLY (secret)
8. It’s amazing hugging diseased tree (6)
WILLOW
WOW (it’s amazing) hugging ILL (diseased)
14. Nothing very rash, yet remarkably rash! (9)
OVERHASTY
O (nothing) + V (very) + (RASH YET)* (*remarkably)
16. Diversion of road with loco in US river (8)
COLORADO
(ROAD with LOCO)* (*diversion of)
17. Nurse brought in a letter for visitor (8)
ATTENDEE
TEND (nurse) brought in (A + TEE (letter))
19. Four involved in ordeal — hardly worth mentioning? (7)
TRIVIAL
IV (four) involved in TRIAL (ordeal)
20. Cross with Gael, not furious (7)
TANGELO
(GAEL NOT)* (*furious)
A citrus hybrid of a tangerine and a pomelo
21. Expression of disappointment is awful where hospital closed off (6)
SHUCKS
SUCKS (is awful) where H (hospital) closed off
22. Superior dirty clothes ending in bin (6)
SNOOTY
SOOTY (dirty) clothes [bi]N (ending in)
25. Problem ending in tears, especially when upset (5)
REBUS
([tear]S (ending in) + UBER (especially))< (<when upset)
As Teacow has it, a gentle and solver-friendly puzzle, but I would have struggled without the crossers falling into place so helpfully.
I’m not so keen on the word “given”, in 26(ac), CROSSFIRE; not “wrong”, as such, just not nice.
Also, I felt a lot of the clues were a bit “samey”, as I was solving, but I didn’t back-check, so that may be nonsense.
At the end of the day, it’s a thumbs up with thanks, to Mudd & Teacow
Ian@1 The use of “given” as a juxtaposition indicator is well-precedented – see the Clue Clinic list here https://clueclinic.com/index.php/juxtaposition-indicators/