Falling behind insulation (7)
I believe the answer is:
lagging
I believe this clue is a double definition.
'falling behind' is the first definition.
('lag' can be a synonym of 'fall behind')
'insulation' is the second definition.
(lagging is a kind of insulation)
(Other definitions for lagging that I've seen before include "dropping behind" , "job in the attic?" , "Insulation -- failing to keep up" , "Getting behind" , "Wrapping used to prevent loss of heat" .)