Former performance? Just so! (5)
I believe the answer is:
exact
'just so' is the definition.
(exact can mean just so or precise)
'former performance?' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'performance?' becomes 'act' (performance is a kind of act).
'ex'+'act'='EXACT'
(Other definitions for exact that I've seen before include "dead" , "Precise - demand" , "Demand and obtain" , "Precisely accurate" , "Strict" .)