Add seasoning to fish - European, posh restaurant might insist on this (5,4)
I believe the answer is:
dress code
'posh restaurant might insist on this' is the definition.
'add seasoning to fish european' is the wordplay.
'add seasoning' becomes 'dress' (e.g. to dress a salad).
'to fish' becomes 'cod' (cod is a kind of fish).
'european' becomes 'e' (abbreviation e.g. EU).
'dress'+'cod'+'e'='DRESS CODE'
(Other definitions for dress code that I've seen before include "Expected standard of attire" , "Convention governing clothes to be warn on a particular occasion" , "Stipulated attire" , "Guidelines" .)