Tree trunk enthusiast (8)
I believe the answer is:
'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).
(Other definitions for chestnut that I've seen before include "Item for stuffing", "Golden-brown or reddish-brown horse", "Tree - reddish-brown", "Old story", "Stale joke or cliche".)