Expression wears thin when spoken (6)
I believe the answer is:
phrase
'expression' is the definition.
(I know that phrase is a type of expression)
'wears thin when spoken' is the wordplay.
'wears thin' becomes 'frays'.
'when spoken' indicates a 'sounds like' (homophone) clue.
'frays' is a homophone of 'PHRASE'.
(Other definitions for phrase that I've seen before include "Saying" , "Words for a seraph" , "Expression without a verb" , "Seraph (anag.)" , "Has rep part of a sentence?" .)