Long garment lacking a measure of thermal protection (3)
I believe the answer is:
tog
'measure of thermal protection' is the definition.
(unit for measuring duvet covers etc)
'long garment lacking a' is the wordplay.
'long garment' becomes 'toga' (long garment worn in ancient Rome).
'lacking' indicates named letters should be taken away.
'toga' with 'a' taken out is 'TOG'.
(Other definitions for tog that I've seen before include "Duvet rating" , "Provide with clothes" , "Quilt rating" , "Unit of measurement of thermal insulation in duvets, coats etc" , "Measure of warmth" .)