The most common and most likely answer to this crossword clue is the 6 letter word ANTIFA
I've seen this crossword clue in The Guardian
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More clues leading to the result ANTIFA
Buddy explains!

The straight part
We know that "Left-wing militants" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case left-wing militants = antifa.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 ANTIFA mean?
ANTIFA proper noun- A decentralised, radical political movement that opposes fascism.
- A local instance of this movement.
- Opposition to fascism
- A person opposed to the tenets of fascism.
- Opposed to the tenets of fascism; antifa.