Mid-week left final orders for fish (4)
I believe the answer is:
eels
'for fish' is the definition.
(eel is a type of fish)
'mid-week left final orders' is the wordplay.
'mid-week' becomes 'ee' (the middle letters of 'week').
'left' becomes 'l' (common abbreviation).
'final' suggests the final letters.
The final letter of 'orders' is 's'.
'ee'+'l'+'s'='EELS'
(Other definitions for eels that I've seen before include "Elongated fish" , "Snakes wriggling in lees" , "eg Conger" , "Conger --; jellied --" , "Sea creatures found in lees" .)