On a motorway, a learner driver won't get far!
I believe the answer is:
'driver won't get far' is the definition.
I can't judge whether this definition defines the answer.
'on a motorway a learner' is the wordplay.
'on' means one lot of letters go next to another.
'a motorway' becomes 'MI' (resembles M1, English motorway).
'a learner' becomes 'l' (as in a learner driver with L-plates on their car).
'mi' next to 'l' is 'MIL'.
Can you help me to learn more?
(Other definitions for mil that I've seen before include "One thousandth of an inch" , "One / one thousand inch" , "tiny distance" , "One thousanth of an inch" .)