Advert extols revealing top (6)
I believe the answer is:
vertex
'top' is the definition.
(I've seen this before)
'advert extols revealing' is the wordplay.
'revealing' says the answer is hidden in the clue.
'VERTEX' is hidden in the letters of 'advert extols'.
(Other definitions for vertex that I've seen before include "Top, apex" , "Corner (of triangle)" , "Summit" , "where sides meet" , "Highest point" .)