Dead end that is restricting motorway flow (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 flow' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'restricting' means one lot of letters goes inside another.
'motorway' becomes 'M' (as in motorway names - Mp, Ms etc).
'flow' becomes 'pass' (I am not sure about this - if you are sure you should believe this answer much more).
'm'+'pass'='mpass'
'ie' going around 'mpass' is 'IMPASSE'.
Can you help me to learn more?
(Other definitions for impasse that I've seen before include "Stalemate situation" , "nothing can be done" , "Maybe gridlock" , "deadlock results" , "Stalling of negotiations" .)