Follow for days? (3)
I believe the answer is:
dog
'follow' is the definition.
(dog can mean to follow or torment)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for dog that I've seen before include "Lassie" , "Cur" , "plague?" , "'Every . . . has its day'" , "Domestic animal" .)