Challenge coming up in better adventure (4)
I believe the answer is:
dare
'challenge' is the definition.
(daring is a kind of challenging)
'coming up in better adventure' is the wordplay.
'coming up' shows that the letters should be reversed in order.
'in' indicates the answer is hidden within the clue.
'erad' can be found hidden inside 'better adventure'.
'erad' written backwards gives 'DARE'.
(Other definitions for dare that I've seen before include "Take a chance - read about it" , "Throw down the gauntlet" , "Challenge of courage" , "Challenge to do something foolhardy" , "Challenge to do something dangerous" .)