Bill returns to boxing, being concerned (6)
I believe the answer is:
caring
'being concerned' is the definition.
(I've seen this in another clue)
'bill returns to boxing' is the wordplay.
'bill' becomes 'ac' (short for account).
'returns' shows that the letters should be reversed in order.
'to boxing' becomes 'ring' (I can't justify this - if you can you should give a lot more credence to this answer).
'ac' backwards is 'ca'.
'ca'+'ring'='CARING'
Can you help me to learn more?
(Other definitions for caring that I've seen before include "Protective or concerned" , "Showing concern" , "Tender, warm" , "Kind-hearted" , "Displaying kindness and concern for others" .)