Muses, say, about part of joint (5)
I believe the answer is:
tenon
'part of joint' is the definition.
(type of wooden joint)
'muses say about' is the wordplay.
'muses say' becomes 'nonet' (a nonet is a group of 9 - there are 9 Greek muses).
'about' says the letters should be written in reverse.
'nonet' in reverse letter order is 'TENON'.
(Other definitions for tenon that I've seen before include "Piece of wood fitting into a mortice" , "End of a piece of wood shaped to form a mortise joint" , "Nonet (anag.)" , "Type of small saw" , "Kind of saw" .)