Delay or block the progress of (6)
I believe the answer is:
impede
I believe this clue is a double definition.
'delay' is the first definition.
(I've seen this in another clue)
'block the progress of' is the second definition.
(I've seen this before)
(Other definitions for impede that I've seen before include "Bar" , "Hold back" , "Delay, be an obstacle" , "Handicap" , "Hinder, be obstructive" .)