Stranger beginning to nibble on a pork pie (5)
I believe the answer is:
alien
'stranger' is the definition.
('alien' can be a synonym of 'stranger')
'beginning to nibble on a pork pie' is the wordplay.
'beginning to' indicates taking the first letters.
'on' says to put letters next to each other.
'pork pie' becomes 'lie' (I've seen this in other clues).
The initial letter of 'nibble' is 'n'.
'a'+'lie'='alie'
'n' put after 'alie' is 'ALIEN'.
(Other definitions for alien that I've seen before include "Creature from another planet" , "Stranger" , "Belonging to a foreign country" , "Clanger?" , "Foreign (from outer space?)" .)