Sound fashionable, say (5,2)
I believe the answer is:
chime in
'say' is the definition.
(thesaurus)
'sound fashionable' is the wordplay.
'sound' becomes 'chime' (chiming is a kind of sounding).
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'chime'+'in'='CHIME IN'
(Other definitions for chime in that I've seen before include "Interject a remark" , "Interrupt" .)