Follow relentlessly (5)
'follow 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 "Hound" , "A pet; its life prov. wretched" , "'Every ... is allowed one bite'" , "Worry" , "Lassie" .)
(Other definitions for hound that I've seen before include "Hunting dog; harass" , "Sir Hugo's killer on Dartmoor" , "despicable creature" , "Chase after" , "Persecute; dog" .)