Quirky person swallowing an unfounded rumour (6)
I believe the answer is:
canard
'unfounded rumour' is the definition.
(I've seen this before)
'quirky person swallowing an' is the wordplay.
'quirky person' becomes 'card'.
'swallowing' is an insertion indicator.
'card' placed around 'an' is 'CANARD'.
(Other definitions for canard that I've seen before include "Untrue story" , "False rumour, originally from France" , "Deliberately misleading fabrication" , "Baseless rumour" , "A false rumour, originally French" .)