Rascal going to island for prawns (6)
I believe the answer is:
scampi
'prawns' is the definition.
(I have seen 'Large prawns' mean 'scampi' so perhaps 'prawns' could also mean 'scampi')
'rascal going to island' is the wordplay.
'rascal' becomes 'scamp' (synonyms).
'going to' means one lot of letters go next to another.
'island' becomes 'i' (geographical abbreviation).
'scamp'+'i'='SCAMPI'
'for' acts as a link.
(Other definitions for scampi that I've seen before include "Dish of cooked shrimps or prawns" , "Cooked Norway lobster" , "Fish dish, often in rings" , "Norway lobsters -- Dublin Bay prawns" , "Dish of crustaceans" .)