Rub out because put in before (5)
I believe the answer is:
erase
'rub out' is the definition.
('erase' can be a synonym of 'rub out')
'because put in before' is the wordplay.
'because' becomes 'as' (I've seen this in other clues).
'put in' indicates putting letters inside.
'before' becomes 'ere' (usually literary**).
'as' put inside 'ere' is 'ERASE'.
(Other definitions for erase that I've seen before include "Remove, wipe out" , "Clear" , "Efface" , "Eradicate" , "Kill" .)