A teeny bit upset by the facts (4)
I believe the answer is:
data
'facts' is the definition.
(I've seen this before)
'a teeny bit upset by' is the wordplay.
'teeny bit' becomes 'tad' (I've seen this in another clue).
'upset' says the letters should be written backwards.
'by' says to put letters next to each other.
'tad' back-to-front is 'dat'.
'a' after 'dat' is 'DATA'.
'the' is the link.
(Other definitions for data that I've seen before include "Raw facts" , "Facts and statistics collected together" , "Listed facts" , "Organised information" , "Numerical facts" .)