A measure of warmth in the direction of drain cover (3,6)
I believe the answer is:
tog rating
'a measure of warmth' is the definition.
(measurement for the warmth of textiles)
'in the direction of drain cover' is the wordplay.
'in the direction of' becomes 'to'.
'drain cover' becomes 'grating' (a drain may be covered by a metal grating).
'to'+'grating'='TOG RATING'
(Other definitions for tog rating that I've seen before include "measure used for heat retention" , "A measure of warmth" .)