Unscrupulous opportunist immersing copper-backed tin in drink (9)
I believe the answer is:
buccaneer
'unscrupulous opportunist' is the definition.
(I've seen this before)
'immersing copper-backed tin in drink' is the wordplay.
I cannot quite understand how this works, but
'tin' could be 'can' (synonyms) and 'can' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for buccaneer that I've seen before include "Unscrupulous opportunist" , "(Caribbean) pirate" , "Sea wolf" , "Corsair" , "Piratical adventurer" .)