Pursue relentlessly (3)
I believe the answer is:
dog
'pursue relentlessly' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for dog that I've seen before include "Contemptible man" , "Worry" , "'Every ... is allowed one bite'" , "'You cannot teach an old ... new tricks'" , "Follow - canine" .)