Set Theory solution
by Kegler

Set Theory solution grid

Across
1DET + ERMINED (TED rev)
7FOCI (1st letters)
9SeT + OLEN (LEON anag)
10GIN R + mUMMY (RING anag)
11ANGER (RANGE anag)
12EM(B)EDDE + D (DEEMED anag)
13CAT + anALYST
14NOCK ("KNOCK" hom)
16P(L)AN
17C + ENTERED
20BATH SOAP (HOP AS BAT anag)
22GROwUP
24PO + SIT(I)ON (IS NOT anag)
25ALER(T)S
26P + SST
27COLLECTION (COOL CLIENT anag)
Down
2EX + TRA (ART rev)
3ENLIGHTEN (IN THEN LEG anag)
4MAN + DRILL
5NUG + G + ETS (GUN rev; SET anag)
6DA + NUB + E (BUN + AD rev)
7FO(U)ND
8COMMEN[C/t]ED
13COL(LA + T)ORS
14N(ARROW)EST
15STAG + NATE
18thEM + PAN + EL
19ZO + DIA + C (rev)
21HO(I)ST
23PAT + I + O