A cause for concern (7,6)
I believe the answer is:
serious matter
'a cause for concern' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'cause for concern' is the wordplay.
'cause' becomes 'serious' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'for' is a charade indicator (letters next to each other).
'concern' becomes 'matter' (both can mean an affair or happening).
'serious'+'matter'='SERIOUS MATTER'
Can you help me to learn more?
(Another definition for serious matter that I've seen is " grave issue".)