Thwart; obstruct (6)
I believe the answer is:
stymie
This clue is a double definition.
'thwart' is the first definition.
(I've seen this in another clue)
'obstruct' is the second definition.
(I know that obstruct can be written as stymie)
(Other definitions for stymie that I've seen before include "Old putting problem" , "Obstruction (originally at golf)" , "'Hinder, obstruct (6)'" , "Block or thwart, say a golf putt" , "Tricky situation" .)