Rub out as in before (5)
I believe the answer is:
erase
'rub out' is the definition.
(I know that rub out can be written as erase)
'as in before' is the wordplay.
'in' indicates putting letters inside.
'before' becomes 'ere' (poetic term for 'before').
'as' going into 'ere' is 'ERASE'.
(Other definitions for erase that I've seen before include "Remove from memory" , "score" , "Put out of mind" , "Clear" , "Eradicate" .)