A viewer's problem crossing road block (6)
I believe the answer is:
stymie
'block' is the definition.
(I know that block can be written as stymie)
'a viewer's problem crossing road' is the wordplay.
'a viewer's problem' becomes 'stye' (problem with the eye, a 'viewer').
'crossing' is an insertion indicator.
'road' becomes 'MI' (resembles M1 motorway).
'stye' enclosing 'mi' is 'STYMIE'.
(Other definitions for stymie that I've seen before include "Block the path of, as on golf green" , "Old putting problem" , "Impede, say golf stroke" , "Tricky situation" , "Hinder the progress of (something)" .)