A Dead Certainty solution
by Kegler

Ten of the answers contain one letter repeated four times; in each case, all four instances get removed before entry.
The removed letters are shown below in red.  In clue order, these letters produce a “four-gone” CONCLUSION.

A Dead Certainty solution grid

Across
1C ARC + TI + C + CIRCLE (CAR anag; IT rev, CLERIC anag)
7TAC + OS (SO CAT rev)
10T(HER)M
12ROTH (hidden)
14PAD + RE
15LE + AVE
16LA + OS
17O GO(O(DAFTER)NO)ON
18FIANCE (IN CAFE anag)
21A + L + TOm
23OF + FENCE
24C + REEP (PEER rev)
26N NONAGENARIANS (AN INSANE GROAN anag)
28LOSES (hidden)
30ANNUL + ARch
32YACHT (CATHY anag)
33BUSHY (2 defs)
35C CON + CUP + ISCENCE (SCIENCE anag)
36STOAt
37VELDT (1st letters)
38SLUR + P
40ARCH (2 defs)
41I + DId + OT (TO rev)
42NOR + THe
43L ELLIPTICALLY (I CALL ILL TYPE anag)
Down
2RHEAS (HEARS anag)
3TEE OFF (TOFFEE anag)
4I'M + AGING
5ROD (2 defs)
6ERE ("ERR" hom)
7TO(L)TEC (OCTET anag)
8CHARLES (RACHEL'S anag)
9SEEN ("SCENE" hom)
10U TU(MU + L)TU + OUSLY (LOUSY anag)
11S CR + ASS + NESS
13I DIV + IN + I + TIES (VIDeos rev)
19ACE + RB
20O OCTOROONS (hidden)
22N N + ON(PER + MAN)E + N,T
25LOANED (ONE LAD anag)
26OSCULAR (CUR ALSO anag)
27INHERIT (even letters)
29ST. + ITCH
30AUELS (hidden)
31LIT + HI + C
34CO + CO + A
35cOVEN
38aSHE
39U(M)P