Criminal con-man in tree (8)
I believe the answer is:
cinnamon
'tree' is the definition.
(cinnamon is a kind of tree)
'criminal con-man in' is the wordplay.
'criminal' indicates anagramming the letters.
'con'+'man'+'in'='conmanin'
'conmanin' anagrammed gives 'CINNAMON'.
(Other definitions for cinnamon that I've seen before include "'Spice in con man, strangely (8)'" , "Spice from dried bark" , "Spicy bark" , "Aromatic tree bark used as a spice" , "The spice is oddly non-manic" .)