Part of stair in tower is eroded (5)
I believe the answer is:
riser
'part of stair' is the definition.
('riser' is part of 'stair')
'in tower is eroded' is the wordplay.
'in' says the answer is hidden in the clue.
'RISER' is hidden in the letters of 'tower is eroded'.
(Other definitions for riser that I've seen before include "Vertical pipe; staircase part" , "Vertical piece between two stair treads" , "Upwardly mobile type" , "person advancing position?" , "Vertical piece between treads of staircase" .)