Cuts gardener's first trees (6)
I believe the answer is:
gashes
'cuts' is the definition.
(I know this)
'gardener's first trees' is the wordplay.
'gardener's first' becomes 'g' (ast letter of 'gardener').
'trees' becomes 'ashes' (ash is a kind of tree).
'g'+'ashes'='GASHES'
(Other definitions for gashes that I've seen before include "Deep cuts or wounds" , "Makes deep cut or wound" , "Deep cuts made with a sharp instrument" .)