Informal drink in many picnic baskets (8)
I believe the answer is:
champers
'informal drink' is the definition.
(informal name for champagne)
'many picnic baskets' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'picnic baskets' becomes 'hampers' (I've seen this in another clue).
'c'+'hampers'='CHAMPERS'
'in' is the link.
(Other definitions for champers that I've seen before include "Might they be extremely eager" , "celebratory drink" , "People who eat" , "Drink casually" , "People putting their teeth in" .)