Share some prosecco, at first, getting sloshed (5)
I believe the answer is:
split
'share' is the definition.
(split can mean someone's share of something)
'some prosecco at first getting sloshed' is the wordplay.
'at first' indicates taking the first letters.
'getting' is a charade indicator (letters next to each other).
'sloshed' becomes 'lit' (I've seen this in another clue).
The first letters of 'some prosecco' is 'sp'.
'sp'+'lit'='SPLIT'
(Other definitions for split that I've seen before include "Cleave lengthwise" , "Cleave or break up" , "run away" , "European city" , "Fork" .)