Lettuce for just three-quarters of the cost? (3)

I believe the answer is:

cos

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'lettuce' is the definition.
(cos lettuce 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" , "Salad ingredient" , "Lettuce with long crisp leaves" , "Greek island and type of lettuce" , "foreign island" .)

I've seen this clue in The Irish Independent.

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