The most common and most likely answer to this crossword clue is the 6 letter word COMMIE
C
O
M
M
I
E
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The straight part
We know that "leftist" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leftist = commie.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 COMMIE mean?
COMMIE noun- One who subscribes to anticapitalism.
- A communist; a person with communist sympathies; a supposed communist infiltrator.
- Communist.
- A Holden Commodore.
- A commercial vehicle.