Flask carried by brother, mostly (7)
I believe the answer is:
thermos
'flask' is the definition.
(I know that thermos is a type of vacuum bottle)
'carried by brother mostly' is the wordplay.
'carried by' indicates a hidden word.
'THERMOS' is hidden in the letters of 'brother mostly'.
(Other definitions for thermos that I've seen before include "Vacuum flask for mothers" , "Container to retain heat" , "Heat-retaining flask for mothers" , "Flask trade mark" .)