Bystander not from one of the two main political groups (5,5)
I believe the answer is:
third party
'bystander' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for third party that I've seen before include "Person not directly involved" , "Liberals once?" , "Kind of insurance" , "Liberal Democrats?" , "Person also involved in a dispute between two others" .)