Her most stylish form keeps things hot (7)
I believe the answer is:
thermos
'keeps things hot' is the definition.
(Thermos flasks keep drinks hot)
'her most stylish form' is the wordplay.
'stylish form' is an anagram indicator.
'her'+'most'='hermost'
'hermost' with letters rearranged gives 'THERMOS'.
(Other definitions for thermos that I've seen before include "Container to retain heat" , "Flask trade mark" , "Heat-retaining flask for mothers" , "Vacuum flask for mothers" .)