A bitter uprising separating two sides on the left? (7)
I believe the answer is:
radical
'the left?' is the definition.
(thesaurus)
'a bitter uprising separating two sides' is the wordplay.
'bitter' becomes 'acid' ('acid' can be a synonym of 'bitter'**).
'uprising' says the letters should be written backwards.
'separating' is an insertion indicator.
'two sides' becomes 'rl'.
'acid' in reverse letter order is 'dica'.
'a'+'dica'='adica'
'adica' going inside 'rl' is 'RADICAL'.
'on' is the link.
(Other definitions for radical that I've seen before include "Deep" , "Going to the root" , "Primitive" , "Political extremist" , "Fundamental - thoroughgoing" .)