Delay, block (6)
I believe the answer is:
impede
I believe this is a double definition.
'delay' is the first definition.
(I've seen this in another clue)
'block' is the second definition.
(I know that block can be written as impede)
(Other definitions for impede that I've seen before include "Interfere with progress" , "Slow" , "Delay; block progress of" , "Block progress of (something)" , "Hinder, be obstructive" .)