Opposite of nadir (6)
I believe the answer is:
zenith
'opposite of nadir' is the definition.
('zenith' is the opposite to 'nadir')
This is all the clue.
(Other definitions for zenith that I've seen before include "Highest point (on celestial sphere)" , "The acme" , "Greatest height" , "Point directly above the nadir" , "Top of battered" .)