Grit big road — put the people off crossing over it (13)
I believe the answer is:
determination
'off crossing over it' is the definition.
I can't tell whether this defines the answer.
'grit big road put the people' is the wordplay.
'grit big' becomes 'deter' (I can't justify this - if you can you should believe this answer much more).
'road' becomes 'MI'.
'put' means one lot of letters go next to another.
'the people' becomes 'nation' (nation is a kind of people).
'deter'+'mi'+'nation'='DETERMINATION'
(Other definitions for determination that I've seen before include "Resoluteness" , "Perseverance" , "Willpower" , "Decision; grit" , "Firm resolve" .)