Navy's football rival (4)
I believe the answer is:
army
'navy's football rival' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for army that I've seen before include "Fighting force" , "See 21" , "A great number (of soldiers?)" , "Body of people organised for war" , "Large number (of ants?)" .)