The most common and most likely answer to this crossword clue is the 6 plus 4 letter word BREATH TEST
I've seen this crossword clue in Financial Times
B
R
E
A
T
H
T
E
S
T
Buddy explains!
The straight part
We know that "Beer that’s drunk by motorist ultimately shown in this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case beer that’s drunk by motorist ultimately shown in this = breath test.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.