Expression wears thin when spoken (6)
I believe the answer is:
Here is my best explanation:
'expression' is the definition.
(phrase is a kind of expression)
'wears thin when spoken' is the subsidiary indication.
'wears thin' becomes 'frays'.
'when spoken' shows a homophone (sound like).
'frays' is a homophone of 'phrase'.
(Other definitions for phrase that I've seen before include "a certain expression", "piece", "partial sentence", "Word (in a particular way)", "Piece of music", "Group of words as unit", "Group of words/notes", "Put into words", "Sequence of words" and "Expression without a verb".)