Rise when about to finish (6)
I believe the answer is:
ascend
'rise' is the definition.
('ascend' can be a synonym of 'rise')
'when about to finish' is the wordplay.
'when' becomes 'as'.
'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 "Move upwards" , "Climb, go up (6)" , "Come up" , "Make the top" , "Climb, rise" .)