The most common and most likely answer to this crossword clue is the 4 letter word MARX
M
A
R
X
More clues leading to the result MARX
- Revolutionary name for famous brothers
- Republican has American backing? Then vote Communist
- Communist points as delivered in broadcast
- Revolutionary listened to records
- Red streaks, did you say?
- Capital fellow is hurt and cross
- Comedian‘s impressions on podcast
- Red stains by one’s ears?
- Socialist hit back wanting favourable sign at the polls
- Karl is correcting student’s work in the auditorium
Buddy explains!

The straight part
We know that "Karl" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case karl = marx.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.