Part of the step in the rear is eroded (5)
I believe the answer is:
riser
'part of the step' is the definition.
('riser' is part of 'step')
'in the rear is eroded' is the wordplay.
'in' indicates a hidden word.
'RISER' is hidden in the letters of 'rear is eroded'.
(Other definitions for riser that I've seen before include "He gets up" , "I get up" , "Vertical pipe; staircase part" , "Upright part of step - vertical pipe" , "an essential to flight" .)