Food provided by a firm during Prohibition (5)
I believe the answer is:
bacon
'food' is the definition.
(bacon is a kind of food)
'a firm during prohibition' is the wordplay.
'a firm' becomes 'co' (a firm is a company).
'during' indicates putting letters inside.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'co' placed inside 'ban' is 'BACON'.
'provided by' acts as a link.
(Other definitions for bacon that I've seen before include "Dried or smoked pig meat" , "Breakfast food" , "Man killed by chicken, some say" , "Sizzling" , "See 9" .)