Prepare to cook fish, heading for event that's black tie, perhaps (5,4)
I believe the answer is:
dress code
'black tie perhaps' is the definition.
(black tie is a formal dress code)
'prepare to cook fish heading for event' is the wordplay.
'prepare to cook' becomes 'dress' (to dress is to clean shellfish before cooking it).
'fish' becomes 'cod' (cod is a kind of fish).
'heading for' says to take the initial letters.
The first letter of 'event' is 'e'.
'dress'+'cod'+'e'='DRESS CODE'
'that's' is the link.
(Other definitions for dress code that I've seen before include "Convention governing clothes to be warn on a particular occasion" , "Expected standard of attire" , "Guidelines" , "Stipulated attire" .)