The most common and most likely answer to this crossword clue is the 5 letter word PINKO
I've seen this crossword clue in The Guardian
P
I
N
K
O
More clues leading to the result PINKO
- Left-leaning type to hold down with powerful punch?
- No! Ukip leader’s going to run as a Liberal?
- Socialist has fix on how fight can be ended
- One of 1 Across perhaps to fix end of boxing match
- Part of joint on floor, a little bit left
- He is leaning to the left-- immobilize with knock out
- Socialist knocks over fine drop of the hard stuff
Buddy explains!

The straight part
We know that "Socialist" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case socialist = pinko.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 PINKO mean?
PINKO noun- A socialist who is not wholly communist.