False communication about union (4)
I believe the answer is:
faux
'false' is the definition.
(I know that false can be written as faux)
'communication about union' is the wordplay.
'communication' becomes 'fax' (I've seen this in another clue).
'about' indicates putting letters inside.
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'fax' enclosing 'u' is 'FAUX'.
(Other definitions for faux that I've seen before include "Artificial" , "Bogus" , "Fake" .)