Wrap up seafood containing nickel (6)
I believe the answer is:
finish
'wrap up' is the definition.
(I've seen this in another clue)
'seafood containing nickel' is the wordplay.
'seafood' becomes 'fish' (fish is a kind of seafood).
'containing' means one lot of letters goes inside another.
'nickel' becomes 'Ni' (Ni is the chemical symbol for nickel).
'fish' going around 'ni' is 'FINISH'.
(Other definitions for finish that I've seen before include "Polish, perhaps" , "Treatment of surface" , "Pefect" , "Deal with" , "curtains" .)