Lettuce, as distinct from 'cabbage', perhaps (3)

I believe the answer is:

cos

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'lettuce' is the definition.
(cos lettuce is a kind of lettuce)

I can't explain the rest of the clue.

(Other definitions for cos that I've seen before include "Chief of Staff" , "Lettuce with long crisp leaves" , "Salad ingredient" , "often associated with sin" , "healthy food" .)

I've seen this clue in the Evening Standard.

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