US representative drills through ecclesiastical site (it's a source of oil) (6)
I believe the answer is:
sesame
'a source of oil' is the definition.
(I've seen this before)
'us representative drills through ecclesiastical site' is the wordplay.
'us representative' becomes 'Sam' (as in Uncle Sam).
'drills through' means one lot of letters goes inside another (inserted letters drill into the word).
'ecclesiastical site' becomes 'see' (area with a cathedral).
'sam' going into 'see' is 'SESAME'.
'it's' acts as a link.
(Other definitions for sesame that I've seen before include "Open ...., says the magician" , "Open . . . . . . is a magic phrase to gain entry" , "Open . . . . . . cries the magician" , "A seed; a TV Street" , "Opening word for a magician" .)