Measure of warmth with a classical garment (4)
I believe the answer is:
toga
'classical garment' is the definition.
(I've seen this before)
'measure of warmth with a' is the wordplay.
'measure of warmth' becomes 'tog' (I've seen this before).
'with' means one lot of letters go next to another.
'tog'+'a'='TOGA'
(Other definitions for toga that I've seen before include "Cloak of ancient Rome" , "Roman citizen's robe" , "Roman male garb" , "Ancient Roman garment" , "Old Roman gown" .)