Challenge pretense behind theatre act? (6)
I believe the answer is:
oppose
'challenge' is the definition.
(I know that challenge is a more specific form of contend)
'pretense behind theatre act?' is the wordplay.
'pretense' becomes 'pose' (pose is a kind of pretense).
'behind' says to put letters next to each other.
'theatre act?' becomes 'op' (I am not sure about this - if you are sure you should believe this answer much more).
'pose' after 'op' is 'OPPOSE'.
Can you help me to learn more?
(Other definitions for oppose that I've seen before include "Bar" , "Act against" , "Refute - confront" , "Take stand against, resist" , "Counter" .)