What does the cryptic crossword clue "Nearly sleep with old flame inside (extremely hot) (7)" mean?

The wordplay uses a container (one word inside another). EX (synonym="old flame") in SIEST (synonym="SIEST") = SEXIEST The answer is SEXIEST.

Nearly sleep with old flame inside (extremely hot) (7)

Guardian Cryptic #29621 | 17 Across | 2025-02-18 flower