Popular revolutionary is, on reflection, one of us (7)
I believe the answer is:
insider
'one of us' is the definition.
(I've seen this before)
'popular revolutionary is on reflection' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'revolutionary' becomes 'red' (red can informally mean communist).
'on reflection' says the letters should be written in reverse.
'red'+'is'='redis'
'redis' written backwards gives 'sider'.
'in'+'sider'='INSIDER'
(Other definitions for insider that I've seen before include "He's in the circle, knows the right people" , "I may know secrets" , "Person privy to secret information" , "Member of clique" , "person with the lowdown" .)