Suggest old tree (5)
I believe the answer is:
opine
'suggest' is the definition.
(to opine is to give or suggest an opinion)
'old tree' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tree' becomes 'pine' (pine tree is a kind of tree).
'o'+'pine'='OPINE'
(Other definitions for opine that I've seen before include "Conjecture" , "State as one's opinion" , "Express or hold view" , "Consider" , "Express one's opinion openly" .)