Cocktail without vermouth, it's magic! (4)
I believe the answer is:
mojo
'it's magic' is the definition.
(I've seen this in another clue)
'cocktail without vermouth' is the wordplay.
'cocktail' becomes 'mojito' (rum-based cocktail).
'without' indicates named letters should be taken away.
'vermouth' becomes 'it' (short for 'Italian vermouth' as in 'gin and it').
'mojito' with 'it' taken out is 'MOJO'.
(Other definitions for mojo that I've seen before include "American charm" , "Magical power" , "Magic charm or power in African-American culture, as heard in R&B music" , "Magical influence" .)