Partially popularise reed pipe (5)
I believe the answer is:
riser
'pipe' is the definition.
(I have seen 'Vertical pipe' mean 'riser' so perhaps 'pipe' could also mean 'riser')
'partially popularise reed' is the wordplay.
'partially' indicates a hidden word.
'RISER' can be found hidden inside 'popularise reed'.
(Other definitions for riser that I've seen before include "Face of a step" , "A bit of a flight" , "Upright piece between steps" , "Front of a step; vertical pipe" , "Upright part of stair step" .)