Source of oil’s identical with large force departing (6)
I believe the answer is:
sesame
'source of oil's' is the definition.
(I've seen this before)
'identical with large force departing' is the wordplay.
I cannot really understand how this works, but
'identical' could be 'same' (similar in meaning) and 'same' is found in the answer.
'force' could be 'e' (E is the symbol of electromotive force) and 'e' is present in the answer.
A single letter 's' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sesame that I've seen before include "'Open . . . . . .'' is a magic phrase to gain access" , "Source of seed and oil" , "Plant yielding seeds used for flavouring bread" , "Password of the Arabian Nights" , "Seed or oil used in cooking" .)