The most common and most likely answer to this crossword clue is the 4 plus 2 plus 3 plus 9 letter word JUMP ON THE BANDWAGON
I've seen this crossword clue in The Times
J
U
M
P
O
N
T
H
E
B
A
N
D
W
A
G
O
N
Buddy explains!
The straight part
We know that "join winning side" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case join winning side = jump on the bandwagon.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.