Dead end that is restricting motorway progress (7)
I believe the answer is:
impasse
'dead end' is the definition.
(I know that dead end can be written as impasse)
'that is restricting motorway progress' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'restricting' indicates putting letters inside.
'motorway' becomes 'M' (as in motorway names - Mp, Ms etc).
'progress' becomes 'pass' (passing is a kind of progressing).
'm'+'pass'='mpass'
'ie' going around 'mpass' is 'IMPASSE'.
(Other definitions for impasse that I've seen before include "cul-de-sac" , "'Deadlock, no progress can be made (7)'" , "Standstill" , "deadlock results" , "Deadlock or stand-off" .)