The most common and most likely answer to this crossword clue is the 7 letter word SOVIETS
S
O
V
I
E
T
S
Buddy explains!
The straight part
We know that "old Reds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old reds = soviets.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 SOVIETS mean?
SOVIETS noun- A workers' council, an institution first formed during the 1905 Russian Revolution and then instituted as the main form of communist government at all levels in the Soviet Union; by extension, a similar organization in early Chinese communism and elsewhere.