Obvious plateau (5)
I believe the answer is:
plain
This clue is a double definition.
'obvious' is the first definition.
('plain' can be similar in meaning to 'obvious')
'plateau' is the second definition.
(thesaurus)
(Other definitions for plain that I've seen before include "Straightforward" , "Simple - flatland" , "Undecorated; flat area" , "not striking" , "Unadorned, not elaborate" .)