Someone with specialised knowledge in drink heard on the radio (7)
I believe the answer is:
insider
'someone with specialised knowledge' is the definition.
'in drink heard on the radio' is the wordplay.
'drink' becomes 'cider' (cider is a kind of drink).
'heard on the radio' shows a homophone (sound like).
'cider' sounds like 'sider'.
'in'+'sider'='INSIDER'
(Other definitions for insider that I've seen before include "Member of charmed circle" , "Accepted member of group - may be privy to scarce information" , "one who's in the know" , "One privy to info by being within organisation" , "One of us" .)