Make a deep cut in good tree (4)
I believe the answer is:
gash
'make a deep cut' is the definition.
'good tree' is the wordplay.
'good' becomes 'g' (abbreviation).
'tree' becomes 'ash' (ash is a kind of tree).
'g'+'ash'='GASH'
'in' acts as a link.
(Other definitions for gash that I've seen before include "Ghastly" , "Gaping wound" , "score heavily" , "Long deep slash" , "Cut; waste (colloq.)" .)