Passage of terminal illness stateside (5)
I believe the answer is:
sinus
'passage' is the definition.
(I know that sinus is a type of passage)
'terminal illness stateside' is the wordplay.
'terminal illness' becomes 'sin' (I can't explain this - if you can you should believe this answer much more).
'stateside' becomes 'US' (short for United States).
'sin'+'us'='SINUS'
'of' is the link.
Can you help me to learn more?
(Other definitions for sinus that I've seen before include "chamber" , "Cavity in the facial bones" , "Head space" , "Channel between brain and skull" , "snotty enclosure" .)