Resolve to put off race across motorway (13)
I believe the answer is:
determination
'resolve' is the definition.
(determination is a kind of resolve)
'put off race across motorway' is the wordplay.
'put off' becomes 'deter' (I've seen this before).
'race' becomes 'nation' (I've seen this before**).
'across' indicates putting letters inside.
'motorway' becomes 'mi' (resembles Mn, English motorway).
'deter'+'nation'='deternation'
'deternation' placed around 'mi' is 'DETERMINATION'.
'to' acts as a link.
(Other definitions for determination that I've seen before include "legal decision" , "Resolution" , "Perseverance" , "Firmness of purpose" , "Decision process" .)