Authority is old and saucy (6)
I believe the answer is:
expert
'authority' is the definition.
(I've seen this before)
'old and saucy' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'and' means one lot of letters go next to another.
'saucy' becomes 'pert' (synonyms).
'ex'+'pert'='EXPERT'
'is' acts as a link.
(Other definitions for expert that I've seen before include "Clever" , "Master" , "Pro" , "Dab hand" , "Specialist, pundit" .)