Charm — it's missing from cocktail (4)
I believe the answer is:
mojo
'charm' is the definition.
(I've seen this before)
'it's missing from cocktail' is the wordplay.
'missing from' suggests deleting specific letters.
'cocktail' becomes 'mojito' (rum-based cocktail).
'mojito' with 'it' taken away is 'MOJO'.
(Other definitions for mojo that I've seen before include "Magical power" , "Magic charm or power in African-American culture, as heard in R&B music" , "Magical influence" , "American charm" .)