Information? A small amount gets sent back (4)
I believe the answer is:
data
'information?' is the definition.
(data are bits of information)
'a small amount gets sent back' is the wordplay.
'a small amount' becomes 'atad' ('a tad' means a small amount of something).
'gets sent back' says the letters should be written in reverse.
'atad' in reverse letter order is 'DATA'.
(Other definitions for data that I've seen before include "Numerical information" , "Series of measurements" , "Results of research" , "Collection of facts, information" , "computer input?" .)