The most common and most likely answer to this crossword clue is the 6 plus 4 letter word CANNON BALL
I've seen this crossword clue in The Sunday Telegraph
C
A
N
N
O
N
B
A
L
L
Buddy explains!
The straight part
We know that "Old projectile" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old projectile = cannon ball.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 CANNON BALL mean?
CANNON BALL noun- (artillery)
- The act of running and jumping in a flexed position into a swimming pool to create a large splash, mimicking the flight and shape of a cannonball.