Cocktail at marriage, endless supply! (9)
I believe the answer is:
margarita
'cocktail' is the definition.
(I know that Margarita is a type of cocktail)
'at marriage endless supply' is the wordplay.
'endless' means to remove the last letter.
'supply' indicates an anagram (in a supple way).
'at'+'marriage'='atmarriage'
'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 "Type of tequila cocktail" , "Caribbean island" , "Tequila and lime cocktail" , "Tequila/citrus cocktail" , "Spanish girl" .)