Strange article masking untruth (5)
I believe the answer is:
alien
'strange' is the definition.
('alien' can be a synonym of 'strange')
'article masking untruth' is the wordplay.
'article' becomes 'an' (indefinite article).
'masking' indicates putting letters inside.
'untruth' becomes 'lie' (lie is a kind of untruth).
'an' enclosing 'lie' is 'ALIEN'.
(Other definitions for alien that I've seen before include "coming from overseas" , "Belonging to a foreign country" , "Clanger?" , "Little green man, perhaps" , "Not native" .)