The most common and most likely answer to this crossword clue is the 8 letter word DEFECTOR
I've seen this crossword clue in The Times
D
E
F
E
C
T
O
R
More clues leading to the result DEFECTOR
- One going over shortcoming, bore at heart
- One that’s crossed over is not one suffering for deceit
- One’ll break faith with failing soldiers
- One who changes sides, finding fault with common soldiers
- Three notes on commonly-spoken bully, person changing sides
- Person who goes over fault with unprotected work
- Det. Force working to expose traitor
- Soldiers in pursuit, failing to get renegade
- Traitor making arrangement to cede fort
Buddy explains!

The straight part
We know that "One going over" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one going over = defector.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 DEFECTOR mean?
DEFECTOR noun- One who defects.