Indent key (3)
I believe the answer is:
'indent key' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tab that I've seen before include "The bill in a restaurant" , "Small flap - bill" , "Tally at bar (colloq.)" , "Text indent key" , "Bar tally; small flap" .)