What some airlines do with tickets is concerning affair (9)
I believe the answer is:
overissue
'what some airlines do with tickets' is the definition.
The definition suggests a singular noun which matches the answer.
'concerning affair' is the wordplay.
'concerning' becomes 'over' (both can mean relating to).
'affair' becomes 'issue' (I can't explain this - if you can you should give a lot more credence to this answer).
'over'+'issue'='OVERISSUE'
'is' is the link.
Can you help me to learn more?
(Other definitions for overissue that I've seen before include "Supply too many" , "Too many notes" .)