Child participating in adventurous programme (5)
I believe the answer is:
sprog
'child' is the definition.
(informal term for a child)
'participating in adventurous programme' is the wordplay.
'participating in' indicates the answer is hidden within the clue.
'SPROG' is hidden in the letters of 'adventurous programme'.
(Other definitions for sprog that I've seen before include "Baby (slang)" , "little one" , "Child; recruit (mil. slang)" , "Infant" , "Child, colloquially" .)