Pole on motorway makes error (7)
I believe the answer is:
mistake
'error' is the definition.
('mistake' can be a synonym of 'error')
'pole on motorway' is the wordplay.
'pole' becomes 'stake' (I've seen this before**).
'on' says to put letters next to each other.
'motorway' becomes 'mi' (resembles Ms, English motorway).
'stake' after 'mi' is 'MISTAKE'.
'makes' acts as a link.
(Other definitions for mistake that I've seen before include "Slip" , "Wrong action or judgment" , "16 down" , "Oversight" , "Error" .)