The most common and most likely answer to this crossword clue is the 4 letter word DARE
I've seen this crossword clue in The Guardian
D
A
R
E
More clues leading to the result DARE
- Challenge accepted by one radical rising up
- Brave, turning up the radio a bit
- District Attorney referring to challenge
- Read about challenge
- Challenge lawyer on limits of role
- Doctor answered challenge
- Challenge poster withdrawn by soldiers
- Read revolutionary’s challenge
- Essential to end Aretha’s challenge
- Reckless daughter, a communist, wicked
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 = dare.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 DARE mean?
DARE noun- A challenge to prove courage.
- The quality of daring; venturesomeness; boldness.
- Defiance; challenge.
- To have enough courage (to do something).
- To defy or challenge (someone to do something)
- To have enough courage to meet or do something, go somewhere, etc.; to face up to
- To terrify; to daunt.
- To catch (larks) by producing terror through the use of mirrors, scarlet cloth, a hawk, etc., so that they lie still till a net is thrown over them.
- To stare stupidly or vacantly; to gaze as though amazed or terrified.
- To lie or crouch down in fear.
- A small fish, the dace.