Cut good tree (4)
I believe the answer is:
gash
'cut' is the definition.
(gashing is a kind of cutting)
'good tree' is the wordplay.
'good' becomes 'g' (abbreviation).
'tree' becomes 'ash' (ash tree is a kind of tree).
'g'+'ash'='GASH'
(Other definitions for gash that I've seen before include "injury" , "surplus to requirements" , "Long deep cut" , "Wide cut" , "Cut deeply" .)