Cocktail is sent over -- beloved cabaret's beginning inside (7)
I believe the answer is:
sidecar
'cocktail' is the definition.
(I know that Sidecar is a type of cocktail)
'is sent over beloved cabaret's beginning inside' is the wordplay.
'sent over' says the letters should be written backwards.
'beloved' becomes 'dear' (synonyms).
'beginning' indicates taking the first letters.
'inside' is an insertion indicator.
The first letter of 'cabarets' is 'c'.
'is' in reverse letter order is 'si'.
'dear' placed around 'c' is 'decar'.
'si'+'decar'='SIDECAR'
(Other definitions for sidecar that I've seen before include "Appendage to motor cycle" , "Tipple" , "Attachment to motor cycle" , "passenger compartment" , "Motorcycle addition" .)