Thanks to Monk for the fun this morning.
A most enjoyable puzzle from Monk. I found this mostly straightforward, with a few head scratchers (8 down took a while to parse – what a clue!), but overall completely fair. A forgiving grid structure also helped keep things moving along nicely. Some wonderful anagrams and surfaces.
The perimeter words all contain a double C, but I haven’t worked out whether there is more to it than that – anyone have more time to spot a pattern perhaps?

(CRITICAL CERN ACT)* (*resolved)
L[oathsom]E (extremely) + PUT< (set, <from the east)
S (second) + HARP (mouth organ) found on CARD (witty guy)
(C (cold) + ROQUE[for]T (blue cheese)), FOR abandoning
WHIT (jot) + MAN (piece, as in chess)
(HOME AGENT)* (*mobile)
HAVE[n] (harbour, almost) brings in E (European)
(OTT (excessive, over the top) + OP (work))< (<held back)
(IS following L (line)) into BALTIC (sea)
(SERIAL [k]I[ller] (second))* (*false)
CHICO (Marx brother) takes in AG (silver)
(“HI” (welcome) + “ALTER” (change)) (“delivered”)
TIT[i]AN (artist, missing an I (island))
([i]POD with PLASTIC MICRO)* (*new)
CD = ‘Corps Diplomatique’ from the French
(E (English) PHD CLAIMS LOCAL)* (*fake)
(IT (Italian) + FORT (stronghold)) in [s]TAT[e] (shelled)
[trus]T (at last) entering PURE (clean) + RU (sport, Rugby Union) after exchange
The exchange being to switch the positions of PURE (clean) and RU (sport) so that RU comes first
TACT (diplomacy) surrounding I (India, NATO alphabet)
(CLEAR THE)* (*ground) around W (wide)
[t]IP (dump) + SWI[t]CH (rod), having no T (time) at all
CHAS (Dave’s sidekick) before M (Mike, NATO alphabet)
Chas ‘n’ Dave were an English musical duo
EX (former) + PEN (US prison, penitentiary) + S[ervic]E (voided) + A + C (Conservative) + COUNT’S (peer’s)
(BEIRUT [c]A[p]I[t]A[l] (regularly))* (*damaged)
R (Romeo, NATO alphabet) nailed by (A + TT (dry, teetotal) + ACTOR (performer))
O (old) + ONAGER< (wild ass, <pulled up)
IDIO[ma]TIC (in a manner of speaking, MA (mother) leaves)
GIR[dle]< (half of, <up)
A cryptic reverse clue
R (run) into COG (something with teeth) + I (one)