The most common and most likely answer to this crossword clue is the 6 plus 6 letter word PRAGUE SPRING
I've seen this crossword clue in The Guardian
P
R
A
G
U
E
S
P
R
I
N
G
More clues leading to the result PRAGUE SPRING
Buddy explains!
The straight part
We know that "when the Reds struggled in Europe" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when the reds struggled in europe = prague spring.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.