Old saucy ace (6)
I believe the answer is:
expert
'ace' is the definition.
(both can mean a master of something)
'old saucy' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'saucy' becomes 'pert' (synonyms).
'ex'+'pert'='EXPERT'
(Other definitions for expert that I've seen before include "One practised" , "Specialist, pundit" , "Person with great knowledge or skill, specialist" , "One has specialist knowledge or ability" , "Clever" .)