Cola is served up in informal gathering (6)
I believe the answer is:
social
'informal gathering' is the definition.
(social event)
'cola is served up' is the wordplay.
'served up' is an anagram indicator.
'cola'+'is'='colais'
'colais' with letters rearranged gives 'SOCIAL'.
'in' acts as a link.
(Other definitions for social that I've seen before include "Gregarious or neighbourly" , "Convivial - party" , "Sort of security" , "Official helping families with problems" , "Of people; gathering" .)