Suggested date under old tree (6)
I believe the answer is:
opined
'suggested' is the definition.
(I've seen this in another clue)
'date under old tree' is the wordplay.
'date' becomes 'd' (common abbreviation eg in DOB, date of birth).
'under' is a charade indicator (letters next to each other) (below, in a down clue).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tree' becomes 'pine' (pine is a kind of tree).
'o'+'pine'='opine'
'd' put after 'opine' is 'OPINED'.
(Other definitions for opined that I've seen before include "Supposed" , "Expressed as one's belief" , "Expressed as one's view" , "Conjectured" , "Gave as one's view" .)