Narrow strip of land connecting areas is therefore restricting motorway (7)
I believe the answer is:
isthmus
'narrow strip of land' is the definition.
(I've seen this before)
'is therefore restricting motorway' is the wordplay.
'therefore' becomes 'thus' (synonyms).
'restricting' is an insertion indicator.
'motorway' becomes 'm'.
'thus' enclosing 'm' is 'thmus'.
'is'+'thmus'='ISTHMUS'
'connecting areas' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for isthmus that I've seen before include "Narrow strip of land connecting two larger land masses" , "Narrow linking piece of land" , "Narrow strip of land in sea" , "Narrow neck of land joining two larger portions" , "Narrow strip linking two larger areas of land" .)