An untruth included by foreigner (5)
I believe the answer is:
alien
'foreigner' is the definition.
('alien' can be a synonym of 'foreigner')
'an untruth included' is the wordplay.
'untruth' becomes 'lie' (lie is a kind of untruth).
'included' is an insertion indicator.
'an' placed around 'lie' is 'ALIEN'.
'by' acts as a link.
(Other definitions for alien that I've seen before include "Foreign - film directed by Ridley Scott" , "Martian, say" , "Not native" , "Otherworldly" , "movie!" .)