Authority is old and saucy (6)
I believe the answer is:
Here is my best explanation:
'authority' is the definition.
(I've seen this before)
'old and saucy' is the wordplay.
'old' becomes 'ex'.
'and' is a charade indicator (letters next to each other).
'saucy' becomes 'pert' ('pert' can be a synonym of 'saucy').
'is' is the link.
(Other definitions for expert that I've seen before include "Skilled workman or specialist", "Dab hand", "Knowledgeable specialist", "The person who knows", "authority", "Having specialised knowledge", "Buff", "I have great knowledge", "Highly skilled or knowledgeable person" and "Pro".)