Become threadbare (4)
I believe the answer is:
fray
'become threadbare' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fray that I've seen before include "Rub off, unravel" , "Tempers may do" , "Brawl - become tattered at the edge" , "Unravel at an edge" , "Donnybrook, free-for-all" .)