Material building up as temperature drops (7)
I believe the answer is:
sacking
'material' is the definition.
(sacking is a kind of material)
'up as temperature drops' is the wordplay.
I cannot quite see how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'building' is the link.
(Other definitions for sacking that I've seen before include "Terminating a person's employment" , "Looting - coarse cloth - dismissal" , "P45" , "Coarse stuff" , "devastating" .)