Sun is rising but cold might cause some irritation here (5)
I believe the answer is:
sinus
'cold might cause some irritation here' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'sun is rising' is the wordplay.
'rising' says the letters should be written in reverse (in a down clue, letters go up).
'sun'+'is'='sunis'
'sunis' in reverse letter order is 'SINUS'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for sinus that I've seen before include "Channel between brain and skull" , "Head space" , "Air cavity in the cranial bones" , "chamber" , "Airspace" .)