The culmination of an attempt to get up in one go (6)
I believe the answer is:
apogee
'the culmination' is the definition.
('apogee' can be a synonym of 'culmination')
'an attempt to get up in one go' is the wordplay.
'an' becomes 'a'.
'attempt' becomes 'go' (go is a kind of attempt**).
'to get up' is a reversal indicator.
'in' indicates putting letters inside.
'one go' becomes 'pee' (both can mean to urinate).
'go' written backwards gives 'og'.
'og' put inside 'pee' is 'pogee'.
'a'+'pogee'='APOGEE'
'of' acts as a link.
(Other definitions for apogee that I've seen before include "Point in planet's orbit when furthest from earth" , "Outermost orbital point" , "crown" , "Pinnacle, highest point" , "Highest point (of orbit)" .)