The most common and most likely answer to this crossword clue is the 9 plus 12 letter word AUTOMATIC TRANSMISSION
I've seen this crossword clue in The Times
A
U
T
O
M
A
T
I
C
T
R
A
N
S
M
I
S
S
I
O
N
Buddy explains!
The straight part
We know that "that assists motorists" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that assists motorists = automatic transmission.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.