Choose to drop even lettuce (3)
I believe the answer is:
cos
'lettuce' is the definition.
(cos is a kind of lettuce)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for cos that I've seen before include "holiday destination" , "often associated with sin" , "Lettuce with long crisp leaves" , "A long-leaved lettuce" , "vegetable?" .)