Vague, during old performance (7)
I believe the answer is:
inexact
'vague' is the definition.
(I've seen this before)
'during old performance' is the wordplay.
'during' becomes 'in' (e.g. during February means in February).
'old' 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, muddled" , "loose" , "Fuzzy - untrue" , "Rough" , "estimated?" .)