The most common and most likely answer to this crossword clue is the 6 plus 9 letter word FELLOW TRAVELLER
F
E
L
L
O
W
T
R
A
V
E
L
L
E
R
More clues leading to the result FELLOW TRAVELLER
Buddy explains!
The straight part
We know that "One supporting Reds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one supporting reds = fellow traveller.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.