The most common and most likely answer to this crossword clue is the 3 plus 2 plus 2 plus 2 letter word NOT UP TO IT
I've seen this crossword clue in The Guardian
N
O
T
U
P
T
O
I
T
More clues leading to the result NOT UP TO IT
Buddy explains!

The straight part
We know that "Unable to meet the challenge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unable to meet the challenge = not up to it.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.