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' says the answer is hidden in the clue.
'ALL' is hidden within 'challenge'.
'taking' is the link.
(Other definitions for all that I've seen before include "Every single one" , "Wholly" , "without exception" , "The works" , "100%" .)