A once lively virtuoso (6)
I believe the answer is:
expert
'virtuoso' is the definition.
(thesaurus)
'a once lively' is the wordplay.
'a once' becomes 'ex' (prefix meaning former or onetime).
'lively' becomes 'pert' (I've seen this in another clue).
'ex'+'pert'='EXPERT'
(Other definitions for expert that I've seen before include "Buff" , "Pro" , "Masterly" , "One who knows his stuff" , "Authority" .)