Insulating material (7)
I believe the answer is:
lagging
'insulating material' is the definition.
(lagging is a kind of insulating material)
This is all the clue.
(Other definitions for lagging that I've seen before include "Not up to date" , "It's used to wrap around boilers, say, to prevent heat-loss" , "Insulating material" , "Wrapping used to prevent loss of heat" , "Getting behind" .)