With which to rub away (6)
I believe the answer is:
eraser
'with which to rub away' is the definition.
I can't tell whether this defines the answer.
'which to rub away' is the wordplay.
'which' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to' means one lot of letters go next to another (I've seen this in other clues).
'rub away' becomes 'erase' (I've seen this in another clue).
'r' after 'erase' is 'ERASER'.
Can you help me to learn more?
(Other definitions for eraser that I've seen before include "Item used for rubbing out" , "A schoolboy's rubber" , "destroyer" , "Implement for rubbing out" , "Obliterator" .)