The most common and most likely answer to this crossword clue is the 16 letter word BUMPER‑TO‑BUMPER
I've seen this crossword clue in The Guardian
B
U
M
P
E
R
â
€
‘
T
O
â
€
‘
B
U
M
P
E
R
Buddy explains!
The straight part
We know that "Unable to move" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unable to move = bumper‑to‑bumper.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.