The most common and most likely answer to this crossword clue is the 7 plus 5 letter word WALKING STICK
I've seen this crossword clue in The Times
W
A
L
K
I
N
G
S
T
I
C
K
Buddy explains!

The straight part
We know that "support for motion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case support for motion = walking stick.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 WALKING STICK mean?
WALKING STICK noun- A tool, such as a cane, used to ease pressure on the legs, and to aid stability, when walking.
- A stick insect (order Phasmida).
- A playing card with the rank of seven.