Source of oil American uncle found in Ely, perhaps (6)
I believe the answer is:
sesame
'source of oil' is the definition.
(I've seen this before)
'american uncle found in ely perhaps' is the wordplay.
'american uncle' becomes 'Sam' (Uncle Sam is a nickname for the USA).
'found in' indicates putting letters inside.
'ely perhaps' becomes 'see' (Ely is a see in the sense of a bishop's area).
'sam' going inside 'see' is 'SESAME'.
(Other definitions for sesame that I've seen before include "which magically opens" , "Part of code for opening" , "Plant grown for its seeds" , "'Open . . . . . .'' is a magic phrase to gain access" , "Opening word for the oilplant?" .)