Rascal gets one type of seafood (6)
I believe the answer is:
scampi
'type of seafood' is the definition.
(I've seen this before)
'rascal gets one' is the wordplay.
'rascal' becomes 'scamp' ('scamp' can be a synonym of 'rascal').
'gets' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'scamp'+'i'='SCAMPI'
(Other definitions for scampi that I've seen before include "Breaded prawns" , "Large prawns cooked in batter" , "Dish of crustaceans" , "Fried seafood" , "Large sauteed prawns" .)