At home, old performance is vague (7)
I believe the answer is:
inexact
'vague' is the definition.
(I've seen this before)
'at home old performance' is the wordplay.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'old' becomes 'ex' (prefix meaning former or onetime).
'performance' becomes 'act' (act is a kind of performance).
'in'+'ex'+'act'='INEXACT'
'is' acts as a link.
(Other definitions for inexact that I've seen before include "Wide of the mark" , "Nice tax (nag.)" , "Fuzzy, muddled" , "Nice tax (anag) - approximate" , "Not precise, like some sciences" .)