It may be high but flat when reached (7)

I believe the answer is:

plateau

I'm a little stuck... Click here to teach me more about this clue!

'it may be high but flat when reached' is the definition.
The definition suggests a singular noun which matches the answer.

This is all the clue.

Can you help me to learn more?

(Other definitions for plateau that I've seen before include "Large level tract of elevated land" , "Raised level area" , "Area of livel high ground" , "Levelling off" , "Elevated flat land" .)

I've seen this clue in The Globe and Mail.

Want a hint initially instead of a full solution? Install my app