The most common and most likely answer to this crossword clue is the 9 letter word OVERREACT
I've seen this crossword clue in Financial Times
O
V
E
R
R
E
A
C
T
More clues leading to the result OVERREACT
- Tucking into ham, soldiers respond too forcibly
- Make mountain of molehill and rate 12 badly
- Maiden possibly about to perform and go over the top?
- Respond immoderately and veto racer after crashing
- Lose sense of proportion when soldiers tuck into ham
- Respond with exaggeration on tucking into ham
- Make much ado about nothing concerning faulty crate
- Most voters care for a change, respond strongly
- Lose it and aim too high, missing last target at the end
Buddy explains!

The straight part
We know that "go over the top" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case go over the top = overreact.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 OVERREACT mean?
OVERREACT verb- To react too much or too intensely.