Young men and topless entertainers after half five cocktail (5,4)
I believe the answer is:
bucks fizz
'cocktail' is the definition.
'young men and topless entertainers after half five' is the wordplay.
'young men' becomes 'bucks' (old-fashioned term for a young man).
'and' is a charade indicator (letters next to each other).
'topless entertainers' becomes 'zz' (I can't explain this - if you can you should believe this answer much more).
'after' means one lot of letters go next to another.
'half' means to take half the letters.
'five' with half the letters taken is 'fi'.
'zz' put after 'fi' is 'fizz'.
'bucks' next to 'fizz' is 'BUCKS FIZZ'.
(Other definitions for bucks fizz that I've seen before include "drink" , "Cocktail consisting of orange juice and champagne" .)