Stage platform (5)
I believe the answer is:
riser
'stage platform' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for riser that I've seen before include "Pipe for upward flow" , "upright portion of step" , "Person getting out of bed" , "Upright piece in stairs" , "Vertical piece between two stair treads" .)