The most common and most likely answer to this crossword clue is the 5 plus 2 letter word TAKEN ON
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More clues leading to the result TAKEN ON
Buddy explains!
The straight part
We know that "Signed up/get challenged" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case signed up/get challenged = taken on.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.