Worry constantly (3)
I believe the answer is:
nag
'worry constantly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nag that I've seen before include "trouble constantly" , "Pester; old horse" , "Badger with constant complaints" , "Complain, worry" , "Complain to or badger incessantly" .)