Cool drink, we hear, for one in the know (7)
I believe the answer is:
insider
'one in the know' is the definition.
(someone with inside information)
'cool drink we hear' is the wordplay.
'cool' becomes 'in' ('in' can mean fashionable or cool).
'drink' becomes 'cider' (alcoholic drink).
'we hear' indicates a 'sounds like' (homophone) clue.
'cider' sounds like 'sider'.
'in'+'sider'='INSIDER'
'for' is the link.
(Other definitions for insider that I've seen before include "One of us" , "Person privy to information within an organisation" , "One with privileged information" , "One privy to restricted information" , "Person privy to secret information" .)