Machine used by learners on the motorway (4)
I believe the answer is:
mill
'machine' is the definition.
(mill is a kind of machine)
'learners on the motorway' is the wordplay.
'learners' becomes 'LL' (L-plates are seen on learner drivers' cars).
'on' is a charade indicator (letters next to each other).
'the motorway' becomes 'MI' (resembles M1, English motorway).
'll' after 'mi' is 'MILL'.
'used by' is the link.
(Other definitions for mill that I've seen before include "Vacuous" , "English philosopher and economist, d. 1873" , "Advocate of the utilitarian" , "Grind (flour)" , "Place for grinding flour" .)