Seafood sensitivity initially affected one (6)
I believe the answer is:
scampi
'seafood' is the definition.
(I've seen this before)
'sensitivity initially affected one' is the wordplay.
'initially' suggests taking the first letters.
'affected' becomes 'camp' (I've seen this before).
'one' becomes 'i' (Roman numeral).
The initial letter of 'sensitivity' is 's'.
's'+'camp'+'i'='SCAMPI'
(Other definitions for scampi that I've seen before include "Dish of fried shrimps or prawns" , "Prawn dish" , "Large prawns as food" , "Fish dish, often in rings" , "Tasty prawns in batter" .)