The most common and most likely answer to this crossword clue is the 6 plus 2 letter word SQUEAK BY
I've seen this crossword clue in Financial Times
S
Q
U
E
A
K
B
Y
More clues leading to the result SQUEAK BY
Buddy explains!

The straight part
We know that "Barely survive/as might a passing mouse?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case barely survive/as might a passing mouse? = squeak by.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.