Harry to keep on complaining (3)
I believe the answer is:
nag
'harry' is the definition.
(both can mean to consistently bother)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for nag that I've seen before include "Henpeck, scold non-stop" , "Mount" , "Annoy the old horse persistently" , "plague" , "Grumble; horse" .)