Challenge what's concealed by Dresden youths (4)
I believe the answer is:
deny
'challenge' is the definition.
(thesaurus)
'concealed by dresden youths' is the wordplay.
'concealed by' indicates a hidden word.
'DENY' is hidden in the letters of 'dresden youths'.
'what's' is the link.
(Other definitions for deny that I've seen before include "Refuse to give" , "Refuse to admit the truth" , "Negate" , "contest" , "Abnegate" .)