Block, encumber (6)
I believe the answer is:
impede
I believe this clue is a double definition.
'block' is the first definition.
(synonyms)
'encumber' is the second definition.
(thesaurus)
(Other definitions for impede that I've seen before include "Delay progress of (something)" , "Hinder or hold up" , "Put obstacle in the way, hinder" , "Hinder, create obstacle" , "Hinder or obstruct, say progress" .)