A little upset by the facts (4)
I believe the answer is:
data
'the facts' is the definition.
(thesaurus)
'a little upset by' is the wordplay.
'little' becomes 'tad'.
'upset' is a reversal indicator.
'by' is a charade indicator (letters next to each other).
'tad' written backwards gives 'dat'.
'a' after 'dat' is 'DATA'.
(Other definitions for data that I've seen before include "Results of research" , "Facts and figures as processed by computer" , "Numerical facts" , "tables, perhaps" , "Listed facts" .)