Everyone taking part in challenge (3)
I believe the answer is:
all
'everyone' is the definition.
(all people)
'part in challenge' is the wordplay.
'part in' indicates the answer is hidden within the clue.
'ALL' is hidden within 'challenge'.
'taking' acts as a link.
(Other definitions for all that I've seen before include "Every single one" , "Wholly" , "without exception" , "The works" , "100%" .)