Rumour king perhaps is entertaining a knight (6)
I believe the answer is:
canard
'rumour' is the definition.
(a canard is a false rumour or story)
'king perhaps is entertaining a knight' is the wordplay.
'king perhaps' becomes 'card' (king is a type of playing card).
'is entertaining' is an insertion indicator (inviting in, as for dinner).
'knight' becomes 'N' (chess abbreviation).
'a'+'n'='an'
'card' enclosing 'an' is 'CANARD'.
(Other definitions for canard that I've seen before include "False rumour, originally from France" , "False rumour or story, from France" , "Ugly rumour" , "Deliberately misleading rumour" , "'False report, hoax (6)'" .)