Bottom pocket filled by flask (10)
I believe the answer is:
nethermost
'bottom' is the definition.
(similar in meaning)
'pocket filled by flask' is the wordplay.
'pocket' becomes 'net' (I've seen this in another clue).
'filled by' indicates putting letters inside.
'flask' becomes 'thermos' (thermos is a kind of flask).
'net' placed around 'thermos' is 'NETHERMOST'.
(Other definitions for nethermost that I've seen before include "at the bottom of the pile" , "located in lowest position" .)