Mother lost small flask (7)
I believe the answer is:
thermos
'flask' is the definition.
(thermos is a kind of flask)
'mother lost small' is the wordplay.
'lost' indicates an anagram.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mother' anagrammed gives 'thermo'.
'thermo'+'s'='THERMOS'
(Other definitions for thermos that I've seen before include "Vacuum flask for mothers" , "Container to retain heat" , "Flask trade mark" , "Heat-retaining flask for mothers" .)