Fail to keep pace (3)
I believe the answer is:
lag
'fail to keep pace' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lag that I've seen before include "Fall behind with a convict" , "Go too slow, fail to keep up" , "Drop behind" , "Convict (slang)" , "Wrap to protect from frost" .)