Mention old tree (5)
I believe the answer is:
opine
'mention' is the definition.
(thesaurus)
'old tree' is the wordplay.
'old' becomes 'o' (abbreviation).
'tree' becomes 'pine' (pine tree is a kind of tree).
'o'+'pine'='OPINE'
(Other definitions for opine that I've seen before include "Suppose" , "Conjecture" , "Give one's view" , "Express or hold view" , "State as one's belief" .)