Highest point of a mountain (6)
I believe the answer is:
summit
'highest point of a mountain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for summit that I've seen before include "Peak or crest" , "High point" , "The highest peak" , "Top of a mountain, or meeting between heads of government" , "Highest point (of hill)" .)