Nag in a fight (8)
I believe the answer is:
warhorse
'nag in a fight' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'nag in' is the wordplay.
I cannot quite see how this works, but
'nag' could be 'horse' (a nag is an old horse) and 'horse' is found in the answer.
The remaining letters 'war' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for warhorse that I've seen before include "something too often played" , "steed?" , "Old soldier" , "Veteran soldier" , "Copenhagen, say" .)