The most common and most likely answer to this crossword clue is the 4 letter word LICK
I've seen this crossword clue in The Daily Telegraph
L
I
C
K
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 = lick.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 LICK mean?
LICK noun- The act of licking; a stroke of the tongue.
- The amount of some substance obtainable with a single lick.
- A quick and careless application of anything, as if by a stroke of the tongue.
- A place where animals lick minerals from the ground.
- A small watercourse or ephemeral stream. It ranks between a rill and a stream.
- A stroke or blow.
- A small amount; a whit.
- An attempt at something.
- A short motif.
- A rate of speed. (Always qualified by good, fair, or a similar adjective.)
- An act of cunnilingus.
- To stroke with the tongue.
- To lap; to take in with the tongue.
- To beat with repeated blows.
- To defeat decisively, particularly in a fight.
- To overcome.
- To perform cunnilingus.
- To do anything partially.
- (of flame, waves etc.) To lap.