Cocktail eschewing vermouth is magic (4)
I believe the answer is:
mojo
'is magic' is the definition.
(I've seen this in another clue)
'cocktail eschewing vermouth' is the wordplay.
'cocktail' becomes 'mojito' (rum-based cocktail).
'eschewing' suggests deleting specific letters.
'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 "Magic charm or power in African-American culture, as heard in R&B music" , "Magical power" , "American charm" , "Magical influence" .)