It may be high but flat when reached (7)
I believe the answer is:
plateau
'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" .)