Salmon and lettuce served up – that has essential vitamin (7)
I believe the answer is:
sockeye
'salmon' is the definition.
(sockeye is a kind of salmon)
'lettuce served up that has essential vitamin' is the wordplay.
'lettuce' becomes 'cos' (cos lettuce is a kind of lettuce).
'served up' shows that the letters should be reversed in order.
'that has' is a charade indicator (letters next to each other).
'essential' becomes 'key' (I've seen this before).
'vitamin' becomes 'e' (E is an example).
'cos' written backwards gives 'soc'.
'soc'+'key'+'e'='SOCKEYE'
'and' is the link.
(Other definitions for sockeye that I've seen before include "Swimmer" , "Type of Pacific salmon" , "Breed of salmon" , "Commercially important salmon" , "Blue-backed salmon" .)