Outlandish falsehood in article (5)
I believe the answer is:
alien
'outlandish' is the definition.
(I've seen this before)
'falsehood in article' is the wordplay.
'falsehood' becomes 'lie' (lie is a kind of falsehood).
'in' means one lot of letters goes inside another.
'article' becomes 'an' (indefinite article).
'lie' placed within 'an' is 'ALIEN'.
(Other definitions for alien that I've seen before include "24a [ESTRANGE]" , "exciting film based in space" , "Foreign (from outer space?)" , "Foreign; extraterrestrial" , "coming from overseas" .)