The most common and most likely answer to this crossword clue is the 9 letter word OUTNUMBER
I've seen this crossword clue in The Daily Telegraph
O
U
T
N
U
M
B
E
R
More clues leading to the result OUTNUMBER
- Have more than revealed by issue
- Have more than old-fashioned dress?
- Count higher than published rank
- Feeling nothing during extreme eclipse
- Be more than an issue after being made public
- Have more bodies than anaesthetist has unconscious first
- Go past snail sadly lost in insurmountable ground
- Tragically insurmountable, if slain_—— wounded
- Be more than relatively stupefied, unconscious at the start
- Overwhelm unfashionable figure
Buddy explains!

The straight part
We know that "Overwhelm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case overwhelm = outnumber.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 OUTNUMBER mean?
OUTNUMBER verb- (stative) to be more in number than somebody or something.