The most common and most likely answer to this crossword clue is the 6 plus 9 letter word SOCIAL DEMOCRATS
I've seen this crossword clue in The Daily Telegraph
S
O
C
I
A
L
D
E
M
O
C
R
A
T
S
More clues leading to the result SOCIAL DEMOCRATS
Buddy explains!
The straight part
We know that "Politicians being moderate left" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case politicians being moderate left = social democrats.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.