Sweetheart knocked back vermouth in second cocktail (6)
I believe the answer is:
mojito
'cocktail' is the definition.
(rum-based cocktail)
'sweetheart knocked back vermouth in second' is the wordplay.
'sweetheart' becomes 'jo' (Scottish term for darling or sweetheart).
'knocked back' is a reversal indicator.
'vermouth' becomes 'it' (short for 'Italian vermouth' as in 'gin and it').
'in' means one lot of letters goes inside another.
'second' becomes 'mo' (both are short periods of time).
'jo' back-to-front is 'oj'.
'oj'+'it'='ojit'
'ojit' going inside 'mo' is 'MOJITO'.
(Other definitions for mojito that I've seen before include "Cuban cocktail with rum as a base" , "Cuban rum-and-lime-based cocktail" , "'Cuban cocktail made from rum, lime etc (6)'" , "Minty cocktail" , "Mexican cocktail made with white rum and lime juice" .)