Onset of illness following rogue food (6)
I believe the answer is:
scampi
'food' is the definition.
(scampi is a kind of food)
'onset of illness following rogue' is the wordplay.
'onset of' indicates taking the first letters.
'following' means one lot of letters go next to another.
'rogue' becomes 'scamp' (I've seen this before).
The first letter of 'illness' is 'i'.
'i' put after 'scamp' is 'SCAMPI'.
(Other definitions for scampi that I've seen before include "Dish of deep-friend prawns" , "Cooked Norway lobster" , "Tasty prawns in batter" , "Shellfish" , "Large sauteed prawns" .)