Flask in bag, farthest down (10)
I believe the answer is:
nethermost
'farthest down' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'flask in bag' is the wordplay.
'flask' becomes 'thermos' (thermos is a kind of flask).
'in' is an insertion indicator.
'bag' becomes 'net' (I've seen this in other clues).
'thermos' inserted inside 'net' is 'NETHERMOST'.
Can you help me to learn more?
(Other definitions for nethermost that I've seen before include "at the bottom of the pile" , "located in lowest position" .)