What does the cryptic crossword clue "Maybe rat’s lair beset by decay (6)" mean?

The wordplay uses a container (one word inside another). DEN (synonym="retreat") in ROT (synonym="ROT") = RODENT The answer is RODENT.

Maybe rat’s lair beset by decay (6)

Guardian Cryptic #29456 | 4 Down | 2024-08-08 flower