Rub out answer that's written the wrong way -- one's beaten
I believe the answer is:
snare drum
'one's beaten' is the definition.
(I've seen this before)
'rub out answer' is the wordplay.
'rub' becomes 'snag' ('snag' can be a synonym of 'rub').
'out' means to remove the last letter.
'answer' becomes 'redrum' (I can't justify this - if you can you should give a lot more credence to this answer).
'snag' with its last letter removed is 'sna'.
'sna'+'redrum'='SNARE-DRUM'
'that's written the wrong way' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for snare drum that I've seen before include "Percussion instrument" , "one's beaten" , "instrument for beating" , "one that gets beaten!" , "One may produce a rattling sound" .)