Crazy upset about union providing bit of information (5)
I believe the answer is:
datum
'bit of information' is the definition.
(I've seen this in another clue)
'crazy upset about union' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'upset' is a reversal indicator.
'about' indicates putting letters inside.
'union' becomes 'tu' (short for trade union).
'mad' written backwards gives 'dam'.
'dam' going around 'tu' is 'DATUM'.
'providing' acts as a link.
(Other definitions for datum that I've seen before include "Fixed starting point for measurements" , "Bit of dope" , "Minimal information" , "Given item" , "One item of information" .)