Comic circulating an unfounded rumour (6)
I believe the answer is:
canard
'unfounded rumour' is the definition.
(I've seen this before)
'comic circulating an' is the wordplay.
'comic' becomes 'card' (old-fashioned term for a humorous person).
'circulating' is an insertion indicator (to circulate is to go round).
'card' going around 'an' is 'CANARD'.
(Other definitions for canard that I've seen before include "Unfounded rumour or story" , "A false rumour from France? (6)" , "A false rumour, originally French" , "Deliberately misleading fabrication" , "False rumour or story, from France" .)