Cardinal tucked into eggs and lettuce (7)
I believe the answer is:
romaine
'lettuce' is the definition.
(romaine lettuce is a kind of lettuce)
'cardinal tucked into eggs' is the wordplay.
'cardinal' becomes 'main' (cardinal can mean very important).
'tucked into' indicates putting letters inside.
'eggs' becomes 'roe' (fish eggs).
'main' inserted inside 'roe' is 'ROMAINE'.
'and' acts as a link.
(Other definitions for romaine that I've seen before include "lettuce in America" , "lettuce in the US" , "Type of lettuce - I arm one (anag)" , "Variety of lettuce" , "Cos lettuce" .)