Co-pilot moans when swimming in drink (12)
I believe the answer is:
cosmopolitan
'drink' is the definition.
(type of cocktail)
'co-pilot moans when swimming' is the wordplay.
'co-pilot' becomes 'copilot' (synonyms).
'when swimming' indicates anagramming the letters.
'copilot'+'moans'='copilotmoans'
'copilotmoans' anagrammed gives 'COSMOPOLITAN'.
'in' acts as a link.
(Other definitions for cosmopolitan that I've seen before include "Familiar with many parts of the world" , "cocktail for a > man of the world" , "all around the world" , "Citizen of the world" , "Worldly; cocktail" .)