Vague during performance, getting old (7)
I believe the answer is:
inexact
'vague' is the definition.
(I've seen this before)
'during performance getting old' is the wordplay.
'during' becomes 'in' (e.g. during February means in February).
'performance' becomes 'act' (act is a kind of performance).
'getting' is a charade indicator (letters next to each other).
'old' becomes 'ex' (prefix meaning former or onetime).
'act' after 'ex' is 'exact'.
'in'+'exact'='INEXACT'
(Other definitions for inexact that I've seen before include "Not precise, like some sciences" , "Nice tax (anag) - approximate" , "Fuzzy, muddled" , "not wholly Right" , "loose" .)