Deceit admitted by an outsider (5)
I believe the answer is:
alien
'outsider' is the definition.
(thesaurus)
'deceit admitted by an' is the wordplay.
'deceit' becomes 'lie' (I've seen this in other clues).
'admitted by' indicates putting letters inside.
'lie' placed into 'an' is 'ALIEN'.
(Other definitions for alien that I've seen before include "Unfamiliar; foreign" , "Foreign; extraterrestrial" , "Offensive" , "coming from overseas" , "Otherworldly" .)