Fish Story solution
by Kegler

Per the unclued phrase, YOU SHOULD HAVE SEEN THE ONE THAT GOT AWAY, fourteen answers have the letters ONE removed before entry into the diagram.

Fish Story solution grid

Across
12HARP + O + {ONE}R
13PLEONASM (SOME PLAN anag)
16ROWBOAT ("BEAU WROTE" spoonerism)
18S + PEAR
20C + HURL
21M(O)A
22PAR(D{ONE})D
24COMM(ISSI){ONE}RS
27B(E)AN
28SEAS + {O + NE}D (DEN rev)
30hAIR
31OHMS ("OMS" hom)
33PA + N
34S.A. + BRA
36{ONE}IDA (hidden)
37STEP + S (rev)
40SPADES (2 defs)
41NECTAR (CRETAN anag)
42N{ONE} + N + FOR + CEMENT
Down
1O + AT + H
2S(P)O{ONE}R
3H(OWL)I + N,G
4UPON (hidden)
5DEaTH + R{ONE}S (SENOR rev)
6H(ORM{ON)E}S
7ANGORA (ON A RAG anag)
8VACATI{ONE}RS (NAIVE ACTORS anag)
9E + SS
10EVER + T
11NOR (last letters)
12HI-C + CUP
14LARIAT (A TRIAL anag)
15R(AD)OME
17BO(SSI)ER (SIS anag)
19PAN + OR + AMA
22PAR(APE)T
23DISH({ONE}ST)Y (STENO anag)
25MEN + ACE
26MAPS (rev)
27B + ADE
29E + X + P + {ON + E}NT (TEN anag)
32H({O,N + E}YD)EW (DYE rev)
34SO + FA
35BERG (hidden)
36INT{ONE} (rev hidden)
38TAN (ANT anag)
39ROT (rev)