Authority of old people holding line (6)
I believe the answer is:
oracle
'authority' is the definition.
(thesaurus)
'old people holding line' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'people' becomes 'race'.
'holding' means one lot of letters goes inside another.
'line' becomes 'l'.
'race' going around 'l' is 'racle'.
'o'+'racle'='ORACLE'
'of' acts as a link.
(Other definitions for oracle that I've seen before include "Divinely-inspired prophecy" , "Place where ancient Greeks consulted a god or prophet" , "Infallible indication" , "She prophesised" , "Medium of divine revelation" .)