Falls behind (4)
I believe the answer is:
lags
'falls behind' is the definition.
('lag' can be a synonym of 'fall behind')
This is all the clue.
(Other definitions for lags that I've seen before include "Drag's one's feet" , "Fails to keep up" , "Slows down and falls behind" , "Convicts" , "Dawdles" .)