In conversation, mention accessory (6)
I believe the answer is:
brooch
'accessory' is the definition.
(I know that brooch is a type of accessory)
'in conversation mention' is the wordplay.
'in conversation' shows a homophone (sound like).
'mention' becomes 'broach' (as in broaching a topic).
'broach' sounds like 'BROOCH'.
(Other definitions for brooch that I've seen before include "Ornamental hinged pin with clasp" , "Ornament with a hinged pin and catch" , "Piece of jewellery" , "Pinned jewellery item" , "Ornament pinned to clothing" .)