It is hidden in discreet additional information (8)
I believe the answer is:
subtitle
'additional information' is the definition.
'it is hidden in discreet' is the wordplay.
'is hidden in' is an insertion indicator.
'discreet' becomes 'subtle' (I've seen this before).
'it' going inside 'subtle' is 'SUBTITLE'.
(Other definitions for subtitle that I've seen before include "Word found at bottom of movie or TV screen" , "Translation of foreign dialogue, in a film say" , "Translate foreign dialogue at bottom of screen" , "A subordinate caption as under foreign film" , "Explanatory legend" .)