Crossword Genius

It can be a source of pain when the sun is up (5)

Ross

I believe the answer is:

sinus

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

'it can be a source of pain when' is the definition.
The definition suggests a singular noun which matches the answer.

'sun is up' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'sun'+'is'='sunis'
'sunis' back-to-front is 'SINUS'.

'the' acts as a link.

Can you help me to learn more?

(Other definitions for sinus that I've seen before include "Air-filled cavity of the skull" , "Anatomical cavity" , "Bodily cavity" , "Airspace" , "Hollow space" .)

I've seen this clue in The Globe and Mail.
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.