Saw nonet assembling (5)
'saw' is the definition.
(I've seen this before)
'nonet assembling' is the wordplay.
'assembling' is an anagram indicator.
'nonet' is an anagram of 'TENON'.
(Other definitions for tenon that I've seen before include "Mortise insert", "Connection", "Piece of wood that fits into a mortise", "Wooden projection shaped to fit in a mortise joint", "Wood fitting into a mortise".)