Back in the Day solution
by Kegler
As indicated by the central entry, YOUNG AT HEART, the twelve answers that have animals at their centers
get those animals' offspring as replacements before entry in the grid.
Across
| 2 | BUT + C(HERS)HOP |
| 10 | CREEd |
| 11 | AUGER (ARGUE anag) |
| 12 | BAA (1st letters) |
| 13 | HI + B + ERNAL (LEARN anag) |
| 15 | L + OFT |
| 16 | GO + D + [M + OTH]ERS {LARVA for MOTH} |
| 18 | C(R)OP |
| 20 | LEIS ("LAYS" hom) |
| 22 | K(AN + S.A.)'S |
| 24 | E + RR |
| 25 | FLO (1st letters) |
| 32 | LEE (2 defs) |
| 33 | SU(PER + FIN)E |
| 34 | P.R. + Y |
| 36 | MIL[L(I + ON] + TH)S {CUB for LION} |
| 38 | AU + GUST |
| 40 | ICERS (rev hidden) |
| 42 | LO[CAT]ED (COLD TEA anag) {KITTEN for CAT} |
| 45 | S[H(A)RE]D {LEVERET for HARE} |
| 46 | MESA (SAME anag) |
| 47 | IMP + ENDS |
|
Down
| 1 | P[RO + BIN]G {CHICK for ROBIN} |
| 2 | B + RIG |
| 3 | TEE (2 defs) |
| 4 | CARLOAD ("CARL OWED" hom) |
| 5 | HUN + AN |
| 6 | R[EEL]S (LEERS anag) {ELVER for EEL} |
| 7 | HBO (last letters) |
| 8 | OA[FISH]LY (SHY FOAL I anag) {FRY for FISH} |
| 9 | PAT[H(OG]EN)S {SHOAT for HOG} |
| 14 | BOO(N + [DOG]GLE)S (LOGGED anag) {PUPPY for DOG} |
| 17 | AIRMEN (MARINE anag) |
| 19 | RAZOR ("RAISER" hom) |
| 21 | S[A(SS)]Y {FOAL for ASS} |
| 23 | S + [COW]L {CALF for COW} |
| 26 | ON(C)E |
| 27 | S + T(O)IC |
| 28 | RETHREAD (HARD TREE anag) |
| 29 | R[OYSTER]S (SORRY SET anag) {SPAT for OYSTER} |
| 30 | B + RISKER |
| 31 | INtUIT |
| 35 | RUB + LE |
| 37 | TheEND |
| 39 | I + TEM (MET rev) |
| 41 | CEDE ("SEED" hom) |
| 43 | OVA (hidden) |
| 44 | D.A.'S (SAD rev) |
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