Lettuce and fish eggs -- most important for inclusion (7)
I believe the answer is:
romaine
'lettuce' is the definition.
(romaine is a kind of lettuce)
'fish eggs most important for inclusion' is the wordplay.
'fish eggs' becomes 'roe' ('roe' can be a synonym of 'fish eggs').
'most important' becomes 'main' (I've seen this before).
'for inclusion' is an insertion indicator.
'roe' enclosing 'main' is 'ROMAINE'.
'and' is the link.
(Other definitions for romaine that I've seen before include "Cos lettuce" , "Variety of lettuce" , "lettuce in America" , "lettuce in the US" , "Type of lettuce - I arm one (anag)" .)