One who gets up part of the stairs (5)
I believe the answer is:
riser
This clue is a double definition.
'one who gets up' is the first definition.
(I've seen this in another clue)
'part of the stairs' is the second definition.
('riser' is part of 'stairs')
(Other definitions for riser that I've seen before include "Vertical piece between stair steps" , "Face of a step" , "Vertical piece between two stair treads" , "Vertical pipe in a building" , "found on the staircase" .)