Very keen to polish up barb (8)
I believe the answer is:
bursting
'very keen to polish up barb' is the definition.
I can't tell whether this defines the answer.
'very keen to polish up' is the wordplay.
'very keen' becomes 'sting' (I can't explain this - if you can you should give a lot more credence to this answer).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'polish' becomes 'rub' (polish meaning rub to make shiny).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'rub' in reverse letter order is 'bur'.
'sting' put after 'bur' is 'BURSTING'.
Can you help me to learn more?
(Another definition for bursting that I've seen is " Breaking open suddenly and violently".)