A sunny one can be a bit shady (3)

I believe the answer is:

day

I'm a little stuck... Click here to teach me more about this clue!

'a sunny one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

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 "Lewis as poet" , "Doris -, US actress" , "24-hour period" , "Born yesterday" , "Dawn to dusk" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.