Seed for cooking oil milled by devotee (9)
I believe the answer is:
groundnut
'seed for cooking oil' is the definition.
(as in groundnut oil)
'milled by devotee' is the wordplay.
'milled' becomes 'ground'.
'by' means one lot of letters go next to another.
'devotee' becomes 'nut' (both can mean an enthusiast).
'ground'+'nut'='GROUNDNUT'
(Other definitions for groundnut that I've seen before include "Peanut" , "edible tuber?" , "something to eat" , "Nibble" , "food" .)