The whole country after one day backing candidate (10)
I believe the answer is:
nomination
'the whole country' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'one day backing candidate' is the wordplay.
'one' becomes 'i' (Roman numeral).
'day' becomes 'mon' (abbreviation for Monday).
'backing' is a reversal indicator.
'candidate' becomes 'nation' (I can't justify this - if you can you should believe this answer much more).
'i'+'mon'='imon'
'imon' backwards is 'nomi'.
'nomi'+'nation'='NOMINATION'
'after' acts as a link.
(Other definitions for nomination that I've seen before include "Naming" , "Putting forward (of a candidate)" .)