The old King, after eating odd pastry dish, is in a worse mood (8)
I believe the answer is:
grumpier
'in a worse mood' is the definition.
I can't judge whether this definition defines the answer.
'the old king after eating odd pastry dish' is the wordplay.
'the old king' becomes 'GR' (Georgius Rex, Latin for King George).
'after eating' indicates putting letters inside.
'odd' becomes 'rum' (rum can mean strange or dubious).
'pastry dish' becomes 'pie' (I've seen this before).
'rum'+'pie'='rumpie'
'gr' going around 'rumpie' is 'GRUMPIER'.
'is' is the link.
Can you help me to learn more?
(Another definition for grumpier that I've seen is " Prime rug (anag.)".)