Tree legislation turns head (6)
I believe the answer is:
walnut
'tree' is the definition.
(walnut is a kind of tree)
'legislation turns head' is the wordplay.
'legislation' becomes 'law' (I've seen this before).
'turns' shows that the letters should be reversed in order.
'head' becomes 'nut' (informal term for somebody's head).
'law' backwards is 'wal'.
'wal'+'nut'='WALNUT'
(Other definitions for walnut that I've seen before include "likely to get cracked at Christmas" , "Fruit tree with valuable wood" , "Hard dark-brown wood used furniture" , "A hardwood" , "Decorative variety of wood" .)