Cryptic crossword clue
Onboard expert with a verdict: brake’s worn 8,6

The most common and most likely answer to this crossword clue is the 8 plus 6 letter word BACKSEAT DRIVER

I've seen this crossword clue in Financial Times
B
A
C
K
S
E
A
T
D
R
I
V
E
R

Buddy explains!

The straight part
We know that "Onboard expert" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case onboard expert = backseat driver.
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.

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