Unusually fine act of warmth (9)
I believe the answer is:
affection
'warmth' is the definition.
(thesaurus)
'unusually fine act of' is the wordplay.
'unusually' is an anagram indicator.
'fine'+'act'+'of'='fineactof'
'fineactof' with letters rearranged gives 'AFFECTION'.
(Other definitions for affection that I've seen before include "Fond attachment" , "Fondness or liking" , "Warm regard" , "Goodwill" , "Fondness, tenderness" .)