The most common and most likely answer to this crossword clue is the 4 plus 2 plus 4 letter word SIDE BY SIDE
I've seen this crossword clue in The Daily Telegraph
S
I
D
E
B
Y
S
I
D
E
More clues leading to the result SIDE BY SIDE
Buddy explains!

The straight part
We know that "shoulder to shoulder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shoulder to shoulder = side by side.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 SIDE BY SIDE mean?
SIDE BY SIDE noun- A type of multipurpose off-highway utility vehicle, with 4-wheels, resembling a club cart or a golf cart
- Adjacent.