Only half the tree is wood? Rubbish! (5)
I believe the answer is:
trash
'rubbish' is the definition.
('trash' can be a synonym of 'rubbish')
'only half the tree is wood?' is the wordplay.
'only half' means to take half the letters.
'is wood?' becomes 'ash' (I've seen this before).
'tree' cut in half is 'tr'.
'tr'+'ash'='TRASH'
(Other definitions for trash that I've seen before include "Worthless people; destroy (US slang)" , "Junk" , "Refuse, waste" , "Wreck; rubbish" , "bunk" .)