Part of a joint (5)
I believe the answer is:
'part of a joint' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tenon that I've seen before include "Projection used in a mortise joint" , "Mortise extension" , "Wood fitting into a mortise" , "Tongue of wood for joint" , "Kind of saw" .)