Go up when about to finish (6)
I believe the answer is:
ascend
'go up' is the definition.
(I know that ascend is a kind of go up)
'when about to finish' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'about' becomes 'c' (abbreviation for 'circa').
'to finish' becomes 'end' ('end' can be a synonym of 'finish').
'as'+'c'+'end'='ASCEND'
(Other definitions for ascend that I've seen before include "Make the top" , "Move upward" , "Take off" , "Climb, go up (6)" , "Climb, rise" .)