Block that is to restrict motorway flow (7)
I believe the answer is:
impasse
'block' is the definition.
(I've seen this before)
'is to restrict motorway flow' is the wordplay.
'to restrict' means to remove the last letter (I've seen 'restricted' mean this (restricted can mean limited or reduced)).
'motorway' becomes 'M' (as in motorway names - Mp, Ms etc).
'flow' becomes 'passe' (I can't explain this - if you can you should believe this answer much more).
'is' with its final letter removed is 'i'.
'i'+'m'+'passe'='IMPASSE'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for impasse that I've seen before include "Unpromising situation" , "failure to progress" , "No progress is possible in this type of situation" , "Dead end" , "'Deadlock, no progress can be made (7)'" .)