The most common and most likely answer to this crossword clue is the 4 plus 3 plus 4 letter word HARD AND FAST
H
A
R
D
A
N
D
F
A
S
T
Buddy explains!
The straight part
We know that "Not moving/like cannonball" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not moving/like cannonball = hard and fast.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 HARD AND FAST mean?
HARD AND FAST adjective- (of rules, etc) strictly maintained