Not initially informed, being elderly (3)
I believe the answer is:
old
'being elderly' is the definition.
('old' can be similar in meaning to 'elderly')
'not initially informed' is the wordplay.
'not initially' means to remove the first letter.
'informed' becomes 'told' (telling is a kind of informing).
'told' with its first letter taken off is 'OLD'.
(Other definitions for old that I've seen before include "Much-used" , "Long in the tooth" , "No longer young" , "out of the ark" , "Getting on" .)