Joker put round a new rumour (6)
I believe the answer is:
canard
'rumour' is the definition.
(a canard is a false rumour or story)
'joker put round a new' is the wordplay.
'joker' becomes 'card' (joker is a type of playing card).
'put round' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'a'+'n'='an'
'card' going around 'an' is 'CANARD'.
(Other definitions for canard that I've seen before include "Unfounded rumour or story" , "False rumour, originally from France" , "A false rumour from France? (6)" , "A false rumour, originally French" , "False rumour also from France" .)