Clean, could become dull in use (9)
I believe the answer is:
unsullied
'clean' is the definition.
('unsullied' can be similar in meaning to 'clean')
'become dull in use' is the wordplay.
'become' indicates an anagram.
'dull'+'in'+'use'='dullinuse'
'dullinuse' is an anagram of 'UNSULLIED'.
'could' acts as a link.
(Other definitions for unsullied that I've seen before include "Pure" , "Not tarnished" , "Spotlessly clean, free from blemishes" .)