Seriously
'seriously' is the definition.
(I know that seriously can be written as in earnest)
This is the entire clue.
'seriously?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for no joke that I've seen before include "Not a trifling matter" , "A serious matter" .)
(Other definitions for joking apart that I've seen before include "If I may be serious" , "But seriously ..." , "Ayckbourn play" .)