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" .)

I've seen this clue in The Globe and Mail.

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