Tree trunk enthusiast (8)
I believe the answer is:
chestnut
'tree' is the definition.
(chestnut tree is a kind of tree)
'trunk enthusiast' is the wordplay.
'trunk' becomes 'chest' (I've seen this before).
'enthusiast' becomes 'nut' (I've seen this before).
'chest'+'nut'='CHESTNUT'
(Other definitions for chestnut that I've seen before include "Horse coat colour" , "Stale joke or cliche" , "Wood" , "Golden-brown or reddish-brown horse" , "Golden to reddish brown colour" .)