Vassal fed, swallowing tablet after pork pie (8)
I believe the answer is:
liegeman
'vassal' is the definition.
(I know that vassal can be written as liegeman)
'fed swallowing tablet after pork pie' is the wordplay.
'fed' becomes 'G-man' (both can mean a US federal agent).
'swallowing' is an insertion indicator.
'tablet' becomes 'e' (E can mean ecstasy or an ecstasy tablet).
'after' says to put letters next to each other.
'pork pie' becomes 'lie' ('pork pie' is Cockney rhyming slang for a lie).
'gman' going around 'e' is 'geman'.
'geman' after 'lie' is 'LIEGEMAN'.
(Other definitions for liegeman that I've seen before include "Vassal, subject" , "Person serving lord" , "Loyal follower" , "A loyal person" .)