Hinder or thwart (6)
I believe the answer is:
stymie
I believe this clue is a double definition.
'hinder' is the first definition.
(I know that hinder can be written as stymie)
'thwart' is the second definition.
(I've seen this in another clue)
(Other definitions for stymie that I've seen before include "Snooker" , "Thwart - block - my site (anag)" , "Thwart, foil" , "Thwart -- my ties (anag)" , "Block or obstruct as in golf (6)" .)