Reveal to be affectation at once (6)
I believe the answer is:
expose
'reveal' is the definition.
(I know that reveal is a more specific form of expose)
'affectation at once' is the wordplay.
'affectation' becomes 'pose' (synonyms).
'at' means one lot of letters go next to another (I've seen this in other clues).
'once' becomes 'ex' (prefix meaning former or onetime).
'pose' after 'ex' is 'EXPOSE'.
'to be' is the link.
(Other definitions for expose that I've seen before include "display" , "Show up" , "Make visible or obvious" , "Abandon" , "article on scandal?" .)