Only a little surprised at astounding facts (4)
I believe the answer is:
data
'facts' is the definition.
(data are bits of information)
'only a little surprised at astounding' is the wordplay.
'only a little' indicates the answer is hidden within the clue.
'DATA' can be found hidden inside 'surprised at astounding'.
(Other definitions for data that I've seen before include "Numerical information" , "Series of measurements" , "intelligence" , "Collection of bits of information" , "Contents of some banks?" .)