What does the cryptic crossword clue "Snack needed by someone overweight in the rain? (3,3)" mean?

The wordplay uses a charade (building blocks). BIG (synonym="BIG") + MAC (synonym="MAC") = BIGMAC The answer is BIGMAC.

Snack needed by someone overweight in the rain? (3,3)

Guardian Cryptic #29569 | 21 Down | 2024-12-18 flower