Fish firm foremost with deliveries (3)
I believe the answer is:
cod
'fish' is the definition.
(type of fish)
'firm foremost with deliveries' is the wordplay.
I cannot quite see how this works, but
'firm' could be 'co' (abbreviation for company) and 'co' is located in the answer.
A single letter 'd' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cod that I've seen before include "mock" , "faked" , "A joke of a fish?" , "Edible fish" , "A fish; a hoax (slang)" .)