Typewriter key (9)
(Another definition for backspace that I've seen is " key used to find character already in?".)
'typewriter key' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tab that I've seen before include "Tally of orders in bar" , "Ring pull" , "Till bill" , "ticket" , "Flap; illicit drug pill (slang)" .)