Cardinal carried by ruminant leaves (7)
I believe the answer is:
romaine
'leaves' is the definition.
(type of lettuce, a leafy vegetable)
'cardinal carried by ruminant' is the wordplay.
'cardinal' becomes 'main' (I've seen this in another clue).
'carried by' indicates putting letters inside.
'ruminant' becomes 'roe' (deer are ruminants).
'main' going into 'roe' is 'ROMAINE'.
(Other definitions for romaine that I've seen before include "Cos lettuce" , "lettuce in the US" , "Variety of lettuce" , "Type of lettuce - I arm one (anag)" , "lettuce in America" .)