Narrow strip of land is accordingly crossing motorway (7)
I believe the answer is:
isthmus
'narrow strip of land' is the definition.
(I've seen this before)
'is accordingly crossing motorway' is the wordplay.
'accordingly' becomes 'thus' (I've seen this in other clues).
'crossing' indicates putting letters inside.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'thus' placed around 'm' is 'thmus'.
'is'+'thmus'='ISTHMUS'
(Other definitions for isthmus that I've seen before include "Narrow strip of land connecting two large masses" , "Narrow linking piece of land" , "eg Corinth, Panama" , "This sum for a narrow piece of land" , "Strip of land flanked by sea" .)