Royals partying are changing the whole calendar (5,4)
I believe the answer is:
solar year
'the whole calendar' is the definition.
The answer and definition can be both related to time as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'royals partying are changing' is the wordplay.
I cannot quite see how this works, but
an anagram of 'royals' is 'solary' which is present in the answer.
an anagram of 'are' is 'ear' which is located in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for solar year that I've seen is " just over 52 weeks".)