The culmination of an attempt to get up in one go (6)
I believe the answer is:
Here is my best explanation:
'the culmination' is the definition.
('apogee' can be a synonym of 'culmination')
'an attempt to get up in one go' is the subsidiary indication.
'an' becomes 'a'.
'attempt' becomes 'go' (go is a kind of attempt).
'to get up' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'one go' becomes 'pee' (both can mean to urinate).
'go' back-to-front is 'og'.
'og' inserted inside 'pee' is 'pogee'.
'of' is the link.
(Other definitions for apogee that I've seen before include "Furthest point of orbit", "Orbit's furthest point", "(Orbit) most distant point from earth", "highly significant point", "Highest or most distant point, or climax", "Point at which a satellite is furthest from the Earth", "Culminating point or pinnacle", "Point in the orbit of the moon that's furthest from the earth", "Pinnacle, highest point" and "The highest point".)