Lawyer takes on the sapper's challenge (4)
I believe the answer is:
dare
'challenge' is the definition.
(daring is a kind of challenging)
'lawyer takes on the sapper's' is the wordplay.
'lawyer' becomes 'da' (D.A. is an example).
'takes on' is a charade indicator (letters next to each other).
'the sapper' becomes 're'.
'da'+'re'='DARE'
(Other definitions for dare that I've seen before include "Be brave enough" , "Challenge, defy, face" , "Risk; challenge" , "Venture, have courage" , "Have the courage to try something" .)