Old people possessing large authority (6)
I believe the answer is:
oracle
'authority' is the definition.
(oracle can mean an authority on a topic)
'old people possessing large' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'people' becomes 'race' (people can mean a national group or race).
'possessing' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'race' enclosing 'l' is 'racle'.
'o'+'racle'='ORACLE'
(Other definitions for oracle that I've seen before include "Person regarded as source of knowledge, like old Greek prophet" , "Sybil" , "Prophet or sage" , "Source of information" , "Augur - prediction" .)