Mother's broken flask (7)
I believe the answer is:
thermos
'flask' is the definition.
(thermos is a kind of flask)
'mother's broken' is the wordplay.
'broken' indicates an anagram.
'mothers' is an anagram of 'THERMOS'.
(Other definitions for thermos that I've seen before include "Flask trade mark" , "Vacuum flask for mothers" , "Container to retain heat" , "Heat-retaining flask for mothers" .)