Info in that addendum taken back (4)
I believe the answer is:
data
'info' is the definition.
(thesaurus)
'in that addendum taken back' is the wordplay.
'in' indicates a hidden word.
'taken back' shows that the letters should be reversed in order.
'atad' can be found hidden inside 'that addendum'.
'atad' written backwards gives 'DATA'.
(Other definitions for data that I've seen before include "Facts - Star Trek android" , "Gathered stats" , "Known facts" , "Contents of some banks?" , "Series of measurements" .)