Rocks on, appearing in revolutionary groups (6)
I believe the answer is:
stones
'rocks' is the definition.
('stone' can be a synonym of 'rock')
'appearing in revolutionary groups' is the wordplay.
'appearing' becomes 'on' (e.g. on stage or on television).
'in' means one lot of letters goes inside another.
'revolutionary' shows that the letters should be reversed in order.
'groups' becomes 'sets' (set is a kind of group**).
'sets' back-to-front is 'stes'.
'on' put into 'stes' is 'STONES'.
'on' acts as a link.
(Other definitions for stones that I've seen before include "Gems -- Mick Jagger's rollers?" , "Balls" , "Rolling rock group" , "lots of rock" , "Rolling ones gather no moss, it's said" .)