A cider's mixed in a cocktail (7)
I believe the answer is:
sidecar
'a cocktail' is the definition.
(I know that sidecar is a type of cocktail)
'a cider's mixed in' is the wordplay.
'mixed' is an anagram indicator.
'in' is an insertion indicator.
'ciders' anagrammed gives 'sidecr'.
'a' put into 'sidecr' is 'SIDECAR'.
(Other definitions for sidecar that I've seen before include "Motorcycle accessory -- cocktail" , "Is raced (anag.)" , "Alcoholic drink" , "Where a passenger may go" , "Appendage to motor cycle" .)