The most common and most likely answer to this crossword clue is the 4 letter word DEFY
I've seen this crossword clue in The Guardian
D
E
F
Y
More clues leading to the result DEFY
- I will leave worship, being brave
- Brave US agent's overthrown last of enemy
- Fed up by the end of Monday’s challenge
- Put iron within 3ft, rising to challenge
- Stand up to democratically elected Free Yemen leaders
- Brave Yankee agent retires
- Fed up youth’s first to resist
- Brave US agent retired on the Fourth of July
- Challenge to make God heartless
- Brave — ultimately afraid, see, of nobody
Buddy explains!

The straight part
We know that "challenge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case challenge = defy.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 DEFY mean?
DEFY noun- A challenge.
- To challenge (someone) or brave (a hazard or opposition).
- To refuse to obey.
- To not conform to or follow a pattern, set of rules or expectations.
- To renounce or dissolve all bonds of affiance, faith, or obligation with; to reject, refuse, or renounce.