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 subsidiary indication.
'old' becomes 'ex'.
'and' means one lot of letters go next to another.
'saucy' becomes 'pert' ('pert' can be a synonym of 'saucy').
'is' is the link.
(Other definitions for expert that I've seen before include "Masterly", "Pro", "Buff", "The person who knows", "Knowledgeable person", "Highly skilled or knowledgeable person", "One has specialist knowledge or ability", "Very knowledgeable person", "Skilled workman or specialist" and "Very skilled or knowledgeable".)