Outlaw climbing no tree (6)
I believe the answer is:
banyan
'tree' is the definition.
(banyan tree is a kind of tree)
'outlaw climbing no' is the wordplay.
'outlaw' becomes 'ban' (banning is a kind of outlawing).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'no' becomes 'nay' (archaic form 'no').
'nay' backwards is 'yan'.
'ban'+'yan'='BANYAN'
(Other definitions for banyan that I've seen before include "Any ban for an Indian tree" , "plant" , "Indian fig tree with self-rooting branches" , "one that does give a fig" , "Crush underfoot" .)