Specialists at old saucy school (7)
I believe the answer is:
experts
'specialists' is the definition.
(I've seen this in another clue)
'at old saucy school' is the wordplay.
'at' is a charade indicator (letters next to each other).
'old' becomes 'ex' (prefix meaning former or onetime).
'saucy' becomes 'pert' (synonyms).
'school' becomes 'S'.
'ex'+'pert' is 'expert'.
'expert'+'s'='EXPERTS'
(Other definitions for experts that I've seen before include "Boffins" , "Aces" , "Buffs" , "Hotshots" , "Specialists" .)