Good wood is cut (4)
I believe the answer is:
gash
'is cut' is the definition.
(a gash is a wound or cut)
'good wood' is the wordplay.
'good' becomes 'g' (abbreviation).
'wood' becomes 'ash' (wood from an ash tree).
'g'+'ash'='GASH'
(Other definitions for gash that I've seen before include "Spare - rubbish" , "injury" , "Deep, jagged tear" , "extra" , "Long deep slash" .)