One in the know (7)
I believe the answer is:
insider
'one in the know' is the definition.
(someone with inside information)
This is the entire clue.
(Other definitions for insider that I've seen before include "member of favoured group?" , "Doing time, right, for this" , "Wrong sort of trading" , "One with privileged info" , "One 24 [IN-SECT], perhaps" .)