Flask going into bag as far down as possible (10)
I believe the answer is:
nethermost
'as far down as possible' is the definition.
(nether means low or at the bottom)
'flask going into bag' is the wordplay.
'flask' becomes 'thermos' (thermos is a kind of flask).
'going into' is an insertion indicator.
'bag' becomes 'net' (I've seen this in other clues).
'thermos' placed inside 'net' is 'NETHERMOST'.
(Other definitions for nethermost that I've seen before include "located in lowest position" , "at the bottom of the pile" .)