Against harsh assessment of some bedding (3,6)
I believe the answer is:
tog rating
'assessment of some bedding' is the definition.
(assessment of a blanket's insulation)
'against harsh' is the wordplay.
'against' becomes 'to' (as in a lean-to, building leaning against anothe).
'harsh' becomes 'grating' (synonyms).
'to'+'grating'='TOG RATING'
(Other definitions for tog rating that I've seen before include "measure used for heat retention" , "A measure of warmth" .)