Challenge against false appearance (8)
I believe the answer is:
confront
'challenge' is the definition.
(I've seen this in another clue)
'against false appearance' is the wordplay.
'against' becomes 'con' (as in pros and cons, for and against).
'false appearance' becomes 'front' (I've seen this in another clue).
'con'+'front'='CONFRONT'
(Other definitions for confront that I've seen before include "Accost, come face to face with" , "Oppose head-on" , "Meet face to face with" , "Face in opposition" , "Square up to" .)