Clock fitted with black wood (6)
I believe the answer is:
timber
'wood' is the definition.
(I've seen this before)
'clock fitted with black' is the wordplay.
'clock' becomes 'timer' (something which times).
'fitted with' is an insertion indicator.
'black' becomes 'b' (abbreviation as in pencil classifications).
'timer' placed around 'b' is 'TIMBER'.
(Other definitions for timber that I've seen before include "board" , "Wood that's been cut and prepared" , "Feller's warning cry" , "Mind the falling tree!" , "Ash perhaps" .)