Rough agent heads up police force linked to gangster's people seeking militant reform (8)
I believe the answer is:
Here is my best explanation:
'rough agent heads up police force linked to gangster's people seeking militant reform' is the definition.
I can't judge whether this definition defines the answer.
'rough agent heads up police force linked to gangster's' is the subsidiary indication.
'heads' indicates taking the first letters.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'police force' becomes 'cid' (I've seen this in other clues**).
'linked to' says to put letters next to each other.
'gangster's' becomes 'als' (Al Capone).
The initial letters of 'rough agent' is 'ra'.
'cid' written backwards gives 'dic'.
(Other definitions for radicals that I've seen before include "People who seek fundamental or drastic reforms" and "Persons requiring drastic reforms or extreme changes".)