What does the cryptic crossword clue "Chests included in fantastic offer, supposedly (7)" mean?

The definition is "Chests". The wordplay uses a hidden word. Hidden in "chests included in fantastiC OFFER, Supposedly" The answer is COFFERS.

Chests included in fantastic offer, supposedly (7)

Guardian Cryptic #28974 | 1 Down | 2023-01-23 flower