The most common and most likely answer to this crossword clue is the 5 plus 5 letter word THIRD PARTY
I've seen this crossword clue in The Sunday Telegraph
T
H
I
R
D
P
A
R
T
Y
Buddy explains!

The straight part
We know that "Kind of policies/political group outside mainstream" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case kind of policies/political group outside mainstream = third party.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does THIRD PARTY mean?
THIRD PARTY adjective- Of or relating to a third party