Barrier breached by workers' alliance — it's a fact (5)
I believe the answer is:
datum
'a fact' is the definition.
(I've seen this before)
'barrier breached by workers' alliance' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'breached by' indicates putting letters inside.
'workers' alliance' becomes 'tu' (abbreviation for Trade Union).
'dam' placed around 'tu' is 'DATUM'.
'it's' is the link.
(Other definitions for datum that I've seen before include "Bit of dope" , "Basic fact" , "Fixed starting point for measurements" , "Premiss, thing taken for granted" , "Something given" .)