Spoil, hinder (6)
I believe the answer is:
impair
This clue is a double definition.
'spoil' is the first definition.
(I know that spoil can be written as impair)
'hinder' is the second definition.
(I've seen this before)
(Other definitions for impair that I've seen before include "Spoil, degrade" , "blunt" , "Courteous man (colloq.)" , "Hurt or handicap" , "Hobble" .)