Tree extremely likely to be old (7)
I believe the answer is:
elderly
'be old' is the definition.
(similar in meaning)
'tree extremely likely' is the wordplay.
'tree' becomes 'elder' (elder is a kind of tree).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'likely' with its centre taken out is 'ly'.
'elder'+'ly'='ELDERLY'
'to' acts as a link.
(Other definitions for elderly that I've seen before include "Somewhat old" , "Of advanced years" , "Aged" , "Getting on in years" , "... or not of tender years" .)