The most common and most likely answer to this crossword clue is the 11 letter word PROVOCATIVE
I've seen this crossword clue in The Times
P
R
O
V
O
C
A
T
I
V
E
Buddy explains!

The straight part
We know that "challenging" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case challenging = provocative.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 PROVOCATIVE mean?
PROVOCATIVE noun- (obsolescent) Something that provokes an appetite, especially a sexual appetite; an aphrodisiac.
- Serving or tending to elicit a strong, often negative sentiment in another person; exasperating.
- Serving or tending to excite, stimulate or arouse sexual interest; sexy.