Drink at marriage, endless bubbly (9)
I believe the answer is:
'drink' is the definition.
(margarita is a kind of drink)
'at marriage endless bubbly' is the wordplay.
'endless' means to remove the last letter.
'bubbly' is an anagram indicator.
'atmarriage' with its final letter taken off is 'atmarriag'.
'atmarriag' with letters rearranged gives 'MARGARITA'.
(Other definitions for margarita that I've seen before include "Stiff drink" , "Type of tequila cocktail" , "Tequila/citrus cocktail" , "Tequila-based cocktail" , "Spanish girl" .)