Sponge off (4)
I believe the answer is:
wipe
'sponge off' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wipe that I've seen before include "Clean with a damp cloth" , "Rub clean with a circular motion" , "Cancel (tape)" , "Remove with cloth" , "Clean with e.g. cloth" .)