Falling behind (7)
I believe the answer is:
lagging
'falling behind' is the definition.
('lag' can be a synonym of 'fall behind')
This is the entire clue.
(Other definitions for lagging that I've seen before include "job in the attic?" , "Not up to date" , "Getting behind" , "thermal insulation?" , "Wrapping used to prevent loss of heat" .)