Chests included in fantastic offer, supposedly (7)
I believe the answer is:
coffers
'chests' is the definition.
(coffer is a kind of chest)
'included in fantastic offer supposedly' is the wordplay.
'included in' says the answer is hidden in the clue.
'COFFERS' is hidden within 'fantastic offer supposedly'.
(Other definitions for coffers that I've seen before include "Supply of funds" , "Funds, financial resources" , "Treasury boxes" , "Treasury box" , "Financial resources or money boxes" .)