Lettuce, as distinct from 'cabbage', perhaps (3)
I believe the answer is:
cos
'lettuce' is the definition.
(cos lettuce is a kind of lettuce)
I can't explain the rest of the clue.
Can you help me to learn more?
(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" .)