Track or follow (4)
I believe the answer is:
tail
I believe this clue is a double definition.
'track' is the first definition.
(I know that track can be written as tail)
'follow' is the second definition.
(both can mean to chase or pursue)
(Other definitions for tail that I've seen before include "bum" , "Back end; follow secretly" , "Elongated posterior part of an animal" , "Queue" , "Hindmost part" .)