Cut a letter on a tree (4)
I believe the answer is:
gash
'cut' is the definition.
(I've seen this before)
'a letter on a tree' is the wordplay.
'a letter' becomes 'g' (**).
'on' means one lot of letters go next to another.
'a tree' becomes 'ash' (ash is a kind of tree).
'g'+'ash'='GASH'
(Other definitions for gash that I've seen before include "Spare - rubbish" , "Gaping wound" , "Wound made by cutting" , "Wide cut" , "Cut; waste (colloq.)" .)