The most common and most likely answer to this crossword clue is the 9 letter word PARAMOUNT
I've seen this crossword clue in The Guardian
P
A
R
A
M
O
U
N
T
More clues leading to the result PARAMOUNT
- Most important soldier on horse, possibly?
- Film studio stage on which a charge is set up
- Unrivalled lover hasn’t finished spiritual texts
- Group filming soldier coming down hill
- Vital soldier dropped next to horse
- Supreme soldier’s horse
- Principal lover cut knot occasionally
- Supreme soldier often up on horse
- Supreme ride for a tough soldier?
- Supreme soldier on horse
Buddy explains!

The straight part
We know that "Group filming" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case group filming = paramount.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 PARAMOUNT mean?
PARAMOUNT noun- A chief or superior.
- Supreme; highest; chief.
- Of the highest importance.