Piece of information union contributed to barrier (5)
I believe the answer is:
datum
'piece of information' is the definition.
(I've seen this before)
'union contributed to barrier' is the wordplay.
'union' becomes 'tu' (I've seen this in other clues).
'contributed to' means one lot of letters goes inside another.
'barrier' becomes 'dam' (dam is a kind of barrier).
'tu' placed into 'dam' is 'DATUM'.
(Other definitions for datum that I've seen before include "Bit of dope" , "Single piece of information" , "Basic fact" , "Fact or stat" , "Given fact" .)