Bother repeatedly (3)
'bother repeatedly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for harass that I've seen before include "Nag" , "Constantly torment" , "Persistently pester" , "press" , "Molest, annoy" .)
(Other definitions for nag that I've seen before include "Old horse or shrewish wife" , "Complain, worry" , "Complaint" , "Poor horse" , "Equine" .)