Old, saucy professional (6)
I believe the answer is:
expert
'professional' is the definition.
(thesaurus)
'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 "Authority" , "Highly skilled or knowledgeable person" , "Pro" , "Specialist, pundit" , "Buff" .)