When the Sun Goes Down single's number 1 in this country (4)
I believe the answer is:
dusk
'when the sun goes down single's number 1 in this country' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'down single's number 1 in this country' is the wordplay.
'down' becomes 'd'.
'single's number s' becomes 's' (1st letter of 'single').
'in' means one lot of letters goes inside another.
'this country' becomes 'UK' (from the perspective of a British newspaper).
's' inserted inside 'uk' is 'usk'.
'd'+'usk'='DUSK'
Can you help me to learn more?
(Other definitions for dusk that I've seen before include "The darker stage of twilight" , "Time towards the end of the day" , "Time of day after sunset" , "Eventide" , "Gloaming" .)