Wear down in fight (4)
I believe the answer is:
fray
This clue is a double definition.
'wear down' is the first definition.
(thesaurus)
'fight' is the second definition.
(fray is a kind of fight)
'in' acts as a link.
(Other definitions for fray that I've seen before include "Become strained" , "Rub off, unravel" , "Wear thin - brawl" , "Unravel at the edges" , "Tempers may do" .)