Baseless rumour (6)
I believe the answer is:
canard
'baseless rumour' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for canard that I've seen before include "A French kind of false rumour" , "A false rumour, originally French" , "'False report, hoax (6)'" , "Deliberately misleading fabrication" , "Ugly rumour" .)