One getting up (5)
I believe the answer is:
riser
'one getting up' is the definition.
(one rising or getting up)
This is the entire clue.
(Other definitions for riser that I've seen before include "Pipe for upward flow" , "Upright part of step - vertical pipe" , "Vertical piece between treads of staircase" , "A bit of a flight" , "Vertical part of a stair or step" .)