The most common and most likely answer to this crossword clue is the 7 letter word TANKING
T
A
N
K
I
N
G
More clues leading to the result TANKING
Buddy explains!

The straight part
We know that "Defeat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defeat = tanking.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 TANKING mean?
TANKING verb- To fail or fall (often used in describing the economy or the stock market); to degenerate or decline rapidly; to plummet.
- To attract the attacks of an enemy target in cooperative team-based combat, so that one's teammates can defeat the enemy in question more efficiently.
- To put (fuel, etc.) into a tank.
- To deliberately lose a sports match with the intent of gaining a perceived future competitive advantage.
- To resist damage; to be attacked without being hurt.
- To contemplate a decision for a long time; to go in the tank.
- To stand; to tolerate.
- A session of being immersed in a tank.