Has it sun only once a week? (3)
I believe the answer is:
day
'week?' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for day that I've seen before include "Particular date" , "Period of time" , "Doris -, US actress" , "Born yesterday" , "Lewis as poet" .)