Observed old tree died (6)
I believe the answer is:
opined
'observed' is the definition.
'old tree died' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tree' becomes 'pine' (pine is a kind of tree**).
'died' becomes 'd'.
'o'+'pine'+'d'='OPINED'
(Other definitions for opined that I've seen before include "Gave as personal view" , "Gave as view or thought" , "Expressed as one's belief" , "Gave as one's view" , "Expressed as one's view" .)