Faithful animal deity rejected (3)
I believe the answer is:
dog
'faithful animal' is the definition.
'deity rejected' is the wordplay.
'deity' becomes 'god' ('god' can be a synonym of 'deity').
'rejected' shows that the letters should be reversed in order.
'god' in reverse letter order is 'DOG'.
(Other definitions for dog that I've seen before include "Sort of collar, fight, days" , "Follow - canine" , "'Every ... is allowed one bite'" , "'You cannot teach an old ... new tricks'" , "Worry" .)