Amount of warmth that her muff contains (5)
I believe the answer is:
therm
'amount' is the definition.
(therm is a kind of amount)
'warmth that her muff contains' is the wordplay.
'contains' indicates a hidden word.
'THERM' is hidden within 'warmth that her muff'.
'of' acts as a link.
(Other definitions for therm that I've seen before include "Unit of heat energy" , "Former gas supply unit" , "Unit of measurement of heat" , "Indicator of heat" , "Unit of heat as in gas supply" .)