Motorway gamble is an error (7)
I believe the answer is:
mistake
'an error' is the definition.
('mistake' can be a synonym of 'error')
'motorway gamble' is the wordplay.
'motorway' becomes 'mi' (resembles M1, English motorway).
'gamble' becomes 'stake' (stake is a kind of gamble).
'mi'+'stake'='MISTAKE'
'is' is the link.
(Other definitions for mistake that I've seen before include "Wrong action or judgment" , "Fault" , "16 down" , "Slip" , "Oversight" .)