Radicals quit side (4)
I believe the answer is:
left
This clue is a triple definition!
'radicals' is the first definition.
(leftwing people)
'quit' is the second definition.
(I've seen this before)
'side' is the third definition.
(as opposed to right)
(Other definitions for left that I've seen before include "Communist?" , "walk-out?" , "Outstanding" , "One < side" , "Socialist direction" .)