Old performance is not approximated in any way (5)
I believe the answer is:
exact
'not approximated in any way' is the definition.
I can't judge whether this definition defines the answer.
'old performance' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'performance' becomes 'act' (act is a kind of performance).
'ex'+'act'='EXACT'
'is' is the link.
Can you help me to learn more?
(Other definitions for exact that I've seen before include "Precise - demand" , "As a right, demand" , "Not approximate; inflict (revenge) on someone" , "Precisely accurate" , "Precisely correct" .)