Mother’s extra heart is a concern (6)
I believe the answer is:
matter
'concern' is the definition.
(both can mean an affair or happening)
'mother's extra heart is a' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'heart' means to look at the middle letters.
'is a' becomes 'ter' (I can't justify this - if you can you should give a lot more credence to this answer).
The centre of 'extra' is 't'.
'ma'+'t'+'ter'='MATTER'
Can you help me to learn more?
(Other definitions for matter that I've seen before include "See 10" , "Subject; be important" , "Substance; be important" , "It occupies space, has substance" , "Concern" .)