Challenge revolutionary retailer advertising clothes (4)
I believe the answer is:
dare
'challenge' is the definition.
(daring is a kind of challenging)
'revolutionary retailer advertising clothes' is the wordplay.
'revolutionary' shows that the letters should be reversed in order.
'clothes' indicates a hidden word (the letters clothe the hidden word).
'erad' can be found hidden inside 'retailer advertising'.
'erad' reversed gives 'DARE'.
(Other definitions for dare that I've seen before include "Be brave enough" , "Throw down the gauntlet" , "Be bold enough (to)" , "Challenge of courage" , "Have sufficient courage" .)