Not quite correct during one-time performance (7)
I believe the answer is:
inexact
'not quite correct' is the definition.
(not completely correct or accurate)
'during one-time performance' is the wordplay.
'during' becomes 'in' (e.g. during February means in February).
'one-time' becomes 'ex' (prefix meaning former or onetime).
'performance' becomes 'act' (act is a kind of performance).
'in'+'ex'+'act'='INEXACT'
(Other definitions for inexact that I've seen before include "Fuzzy - untrue" , "Fuzzy, muddled" , "Not quite right" , "Not entirely accurate" , "estimated?" .)