Amount of money, including flyer, that’s found in chest (7)
I believe the answer is:
sternum
'that's found in chest' is the definition.
(part of the chest)
'amount of money including flyer' is the wordplay.
'amount of money' becomes 'sum' ('sum' can be a synonym of 'amount of money').
'including' is an insertion indicator.
'flyer' becomes 'tern'.
'sum' enclosing 'tern' is 'STERNUM'.
(Other definitions for sternum that I've seen before include "Chest bone" , "Munster (anag.)" , "big bone" , "somewhere in chest" , "Human bone -- ten rums (anag)" .)