Popular drink, we’re told, for one in the know (7)
I believe the answer is:
insider
'one in the know' is the definition.
(someone with inside information)
'popular drink we're told' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'drink' becomes 'cider' (cider is a kind of drink).
'we're told' shows a homophone (sound like).
'cider' is a homophone of 'sider'.
'in'+'sider'='INSIDER'
'for' is the link.
(Other definitions for insider that I've seen before include "Person privy to secret information" , "in prison?" , "person with the lowdown" , "member of favoured group?" , "Accepted member of group, in the know" .)