Impetuous forester's last tree (4)
I believe the answer is:
rash
'impetuous' is the definition.
(I know that impetuous can be written as rash)
'forester's last tree' is the wordplay.
'last' says to take the final letters.
'tree' becomes 'ash' (ash is a kind of tree).
The final letter of 'forester' is 'r'.
'r'+'ash'='RASH'
(Other definitions for rash that I've seen before include "Red area on skin" , "Unwisely incurring danger" , "complaint" , "Impetuous; skin ailment" , "Over-hasty, reckless" .)