Woodman in favour of new trees (8)
I believe the answer is:
forester
'woodman' is the definition.
(I've seen this before)
'in favour of new trees' is the wordplay.
'in favour' becomes 'for' (as in being for a particular cause).
'of new' indicates an anagram (I've seen 'new' mean this).
'trees' is an anagram of 'ester'.
'for'+'ester'='FORESTER'
(Other definitions for forester that I've seen before include "Woodman" , "Hornblower novelist" , "One in charge of woodlands" , "Tree expert" , "One works among trees" .)