Extremely low number returned drinking flask (10)
I believe the answer is:
nethermost
'extremely low' is the definition.
'number returned drinking flask' is the wordplay.
'number' becomes 'ten' (ten is a kind of number).
'returned' says the letters should be written in reverse.
'drinking' means one lot of letters goes inside another.
'flask' becomes 'thermos' (thermos is a kind of flask).
'ten' back-to-front is 'net'.
'net' enclosing 'thermos' is 'NETHERMOST'.
(Other definitions for nethermost that I've seen before include "located in lowest position" , "at the bottom of the pile" .)