In fact, her most insulated soup holder? (7)
I believe the answer is:
thermos
'soup holder?' is the definition.
(a thermos might contain soup)
'in fact her most insulated' is the wordplay.
'in' indicates the answer is hidden within the clue.
'THERMOS' is hidden within 'fact her most insulated'.
(Other definitions for thermos that I've seen before include "Flask trade mark" , "Heat-retaining flask for mothers" , "Vacuum flask for mothers" , "Container to retain heat" .)