The most common and most likely answer to this crossword clue is the 8 letter word MASSACRE
I've seen this crossword clue in The Guardian
M
A
S
S
A
C
R
E
More clues leading to the result MASSACRE
- Service area, carnage
- Heavily defeat male, since all but untouchable
- Crush in service area
- Carnage in service area
- Hiding, maybe, in service area
- Butcher’s service, outstanding, Queen admitted
- Service expert welcoming Queen’s total defeat
- Butcher’s block as carver regularly used
- Service winner overcomes resistance in overwhelming victory
- Wipe out an extensive area
Buddy explains!

The straight part
We know that "overwhelming victory" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case overwhelming victory = massacre.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 MASSACRE mean?
MASSACRE noun- The killing of a considerable number (usually limited to people) where little or no resistance can be made, with indiscriminate violence, without necessity, and contrary to civilized norms.
- Murder.
- Any overwhelming defeat, as in a game or sport.
- To kill in considerable numbers where little or no resistance can be made, with indiscriminate violence, without necessity, and contrary to civilized norms. (Often limited to the killing of human beings.)
- To win so decisively it is in the manner of so slaughtering one's opponent.
- To give a performance so poorly it is in the manner of so slaughtering the musical piece, play etc being performed.