Heads below box trees (9)
I believe the answer is:
chestnuts
'trees' is the definition.
(chestnut is a kind of tree)
'heads below box' is the wordplay.
'heads' becomes 'nuts' (I've seen this before).
'below' means one lot of letters go next to another.
'box' becomes 'chest' (chest is a kind of box**).
'nuts' after 'chest' is 'CHESTNUTS'.
(Other definitions for chestnuts that I've seen before include "Tree fruits called conkers - some are edible" , "Common tree-fruits, sometimes roasted" , "often retold stories" , "horses" .)