The most common and most likely answer to this crossword clue is the 9 letter word OVEREXERT
I've seen this crossword clue in The Times
O
V
E
R
E
X
E
R
T
More clues leading to the result OVEREXERT
- Old audit including King and Queen's tax
- English monarch seen in public, showing strain
- English king cutting public tax beyond what's reasonable
- Capturing King and Queen, old, old soldier’s to stress too much
- Old queen for whom clear bandages stretch too much
- Excessively strain English king in public
- Work too hard crossing river, right?
- Hedging before vote, public work too hard
- Retiring king crosses gorge for a stretch too far
- About to come back once in the open and try too hard
Buddy explains!

The straight part
We know that "tax" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tax = overexert.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 OVEREXERT mean?
OVEREXERT verb- To exert (oneself) to an excessive degree