Pork pie eaten by an extraterrestrial (5)
I believe the answer is:
alien
'extraterrestrial' is the definition.
('alien' can be a synonym of 'extraterrestrial')
'pork pie eaten by an' is the wordplay.
'pork pie' becomes 'lie' (I've seen this in other clues).
'eaten by' is an insertion indicator.
'lie' inserted within 'an' is 'ALIEN'.
(Other definitions for alien that I've seen before include "Belonging to a foreign country" , "One is from outer space, perhaps" , "coming from overseas" , "extraneous" , "of foreigners" .)