The most common and most likely answer to this crossword clue is the 9 plus 9 letter word CELESTIAL MECHANICS
I've seen this crossword clue in Independent
C
E
L
E
S
T
I
A
L
M
E
C
H
A
N
I
C
S
Buddy explains!
The straight part
We know that "Bodies in motion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bodies in motion = celestial mechanics.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.