Flask placed in bag at the bottom (10)
I believe the answer is:
nethermost
'the bottom' is the definition.
(I've seen this before)
'flask placed in bag' is the wordplay.
'flask' becomes 'thermos' (thermos is a kind of flask).
'placed in' means one lot of letters goes inside another (I've seen this in other clues).
'bag' becomes 'net' (I've seen this in other clues).
'thermos' put within 'net' is 'NETHERMOST'.
'at' acts as a link.
(Other definitions for nethermost that I've seen before include "at the bottom of the pile" , "located in lowest position" .)