Key error admitted by party-pooper (4)
I believe the answer is:
typo
'key error' is the definition.
(error made when typing keys)
'admitted by party-pooper' is the wordplay.
'admitted by' indicates a hidden word.
'TYPO' is hidden in the letters of 'party pooper'.
(Other definitions for typo that I've seen before include "Printing error (colloq.)" , "Misprint" , "What's not right" , "Finger problem" , "Small printing error" .)