The most common and most likely answer to this crossword clue is the 10 letter word TRANSITORY
I've seen this crossword clue in The Daily Telegraph
T
R
A
N
S
I
T
O
R
Y
More clues leading to the result TRANSITORY
- Unusual strain on defender of the status quo that’s not permanent
- Reorganisation of trains by Conservative is short-lived
- Brief managed seat held by Conservative
- Passing in van, sharing time with politician
- Short-lived politician suppressed by terrible strain
- Go without terrible rations disappearing quickly
- Brief strain troubled politician
- Brief Conservative following terrible strain
- Passing star in ground with politician
Buddy explains!

The straight part
We know that "Passing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case passing = transitory.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.
What does TRANSITORY mean?
TRANSITORY adjective- Lasting only a short time; temporary.
- Of an action: that may be brought in any county