Impeccable performance no longer? (5)
I believe the answer is:
exact
'impeccable' is the definition.
(thesaurus)
'performance no longer?' is the wordplay.
I cannot quite understand how this works, but
'performance' could be 'act' (act is a kind of performance) and 'act' is found within the answer.
'no longer?' could be 'ex' (prefix meaning former or onetime) and 'ex' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for exact that I've seen before include "Demand, extort" , "Strict" , "Precisely accurate" , "accurate in all details" , "Correct; insist on" .)