Because it's said to be a sort of lettuce (3)
I believe the answer is:
cos
This clue might be a double definition.
'because' is the first definition.
(I've seen this in other clues)
'lettuce' is the second definition.
(cos is a kind of lettuce)
'it's said to be a sort of' is the link.
I am not very happy about this link. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for cos that I've seen before include "something of a salad" , "Island in Mediterranean" , "healthy food" , "Salad plant" , "Variety of lettuce" .)