Source of radiation is up — it's a problem for airways (9)
I believe the answer is:
sinusitis
'source of radiation' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'up it's a problem for airways' is the wordplay.
I cannot quite see how this works, but
'it's' could be 'tis' (I've seen this before) and 'tis' is present in the answer.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for sinusitis that I've seen before include "Inflammation of nasal cavities" , "Inflammation of head cavities" , "nasal problem" , "Inflammation in the skull" , "Nasal membrane inflammation" .)