The most common and most likely answer to this crossword clue is the 5 plus 6 plus 4 letter word CROSS SWORDS WITH
I've seen this crossword clue in The Daily Telegraph
C
R
O
S
S
S
W
O
R
D
S
W
I
T
H
More clues leading to the result CROSS SWORDS WITH
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 = cross swords with.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.