Classic IO.
I enjoyed this very much. IO always poses a tricky challenge, and this is no exception. The trickier the mental workout, the greater the sense of satisfaction when the penny drops.
If you have a slightly different take on the parsing, do leave a comment. Many thanks to the setter.

THEBES (old city) needs TIN (money); ([b]U[t] (essentially + SIN (wrong)) to dig THEBES (it again) + S (son)
VE[t] (examine) parts UN[t]IL (before); (two Ts (tenors) retiring)
PING (system test) involving (ELI (Sam’s teacher, biblical) + CAN (may) + CROSS (block))
LID (hat); (I’M (one’s) + P (pastor)) is wearing
M[oorfield] (‘s chief) + IS + T (tense) keeping GRAF (count)
‘Graf’ is a German title of nobility equivalent to a count
Cryptic definition
A well is also a source of oil, and a ‘to-do’ is a scuffle
UP (completely) blocked by LINE (row)
For ‘up’ think ‘drink up’ or ‘use up’ to mean ‘completely’
OF (during); (MANY THOUSANDS)* (*wandering, around)
[sel]FIES TA[ken] (to capture)
(WASN’T RECORDED)* (*sadly); (HERE (where I am) + FOR (because of)) launches
In (HOW (which style) + I); USE (habitually employ) + FE (iron) – &lit
BLIN[d] (undiscerning, D (day) not observed) + I (institute)
Cryptic definition
“SCENE” with/and “KNOT” (“heard”)
Double definition
I.D.s (to check someone’s identity); Iain Duncan Smith, Tory leader often referred to by his initials
(TORTUOUS CLUE)* (*edited) + ELA (what Morse needed to think about) entering
ELA = English Language Arts; Morse having invented a ‘language’ system
DAVE’S (channel’s, cycling); PORE< (opening, <up) tours
The UK has a television channel called ‘Dave’
“MEAN” (humble, “broadcaster’s”)
COMF[y] (completely at ease, almost) with FOE< (adversary, <retired) in
(IS OPENLY A)* (*deviant)
(N (new) CAFE FRY)* (*up) with [t]E[a] (unlimited)
Cryptic definition
MA IN is the opposite (counter) of POP OUT
Double definition
20 (all fingers and toes maybe) submerged under H (hot)
IO includes a number in the grid – controversial!
It’s a Z in the grid image as numbers aren’t accepted in my solving tool…
Dave changed its name to U&Dave more than 2 years ago so 7d is a bit out of date.
Managed to finish but a few were only partially parsed (eg ELA in 6d) and needed help finding the old county.
Quite liked the number in H2O. There was an FT crossword some years ago with a nina along the lines ‘Cinephile is 90 years old today’ with solutions ‘satur9 (saturnine)’ and ‘ground 0 (zero)’ to give the ‘90’.
SEEN AND NOT HEARD was my favourite. Maybe Henri could have helped with some of the others, but Oriel puts it very well. It’s satisfying when you finally get there.
6D: I think the reference is to Inspector Morse, and the element of wordplay is ALE inverted (about). Morse would go to the pub to think about solving a case.
7D: I could not parse this. NHO Dave.
Thanks for a great blog , never easy to explain all of this .
The long entries helped a lot giving many letters and I found the definitions rather friendly .
6D: I agree with Cineraria.
15A: Solvers outside the UK might not know that Moorfields is a leading eye hospital (to which I have reason to be grateful).
I checked this after enjoying the usual IO quality-just to check if a 2 was acceptible as the ans was H2O
I never heard that using a numbers was out of bounds
Rock on JH!