Deceive badly, admitting new testimony (8)
I believe the answer is:
evidence
'testimony' is the definition.
(testimony is evidence given in court)
'deceive badly admitting new' is the wordplay.
'badly' indicates anagramming the letters (letters in the wrong order).
'admitting' is an insertion indicator (admit can mean to welcome in or accept).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'deceive' is an anagram of 'evidece'.
'evidece' enclosing 'n' is 'EVIDENCE'.
(Other definitions for evidence that I've seen before include "It's offered in court" , "support" , "Data" , "authentication" , "Confirmation" .)