Clearly not mountainous (7)
I believe the answer is:
plainly
'clearly' is the definition.
('plainly' can be a synonym of 'clearly')
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for plainly that I've seen before include "Clearly" , "In unconcealed fashion" , "obviously" , "Simply" , "... without embellishment" .)