Seafood affected one after start of show (6)
I believe the answer is:
scampi
'seafood' is the definition.
(I've seen this before)
'affected one after start of show' is the wordplay.
'affected' becomes 'camp' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'after' means one lot of letters go next to another.
'start of' indicates taking the first letters.
The first letter of 'show' is 's'.
'camp'+'i'='campi'
'campi' after 's' is 'SCAMPI'.
(Other definitions for scampi that I've seen before include "Fish dish, often in rings" , "Tasty prawns in batter" , "Dish of cooked shrimps or prawns" , "Dish of deep-friend prawns" , "Dish of fried shrimps or prawns" .)