Sweetheart! The ruler is not serious (6)
I believe the answer is:
joking
'not serious' is the definition.
(I've seen this before)
'sweetheart the ruler' is the wordplay.
'sweetheart' becomes 'jo' (Scottish term for darling or sweetheart).
'the ruler' becomes 'king' (king is a kind of ruler).
'jo'+'king'='JOKING'
'is' is the link.
(Other definitions for joking that I've seen before include "Kidding" , "Not serious" , "Jesting, being humorous" , "funny" , "Making quips" .)