Aging tree, largely hollow (7)
I believe the answer is:
elderly
'aging' is the definition.
(thesaurus)
'tree largely hollow' is the wordplay.
'tree' becomes 'elder' (elder is a kind of tree**).
'hollow' suggests removing the centre.
'largely' with its middle removed is 'ly'.
'elder'+'ly'='ELDERLY'
(Other definitions for elderly that I've seen before include "Aged" , "Mature" , "Long in the tooth" , "Quite old" , "like a rich attorney's ugly daughter" .)