A pair royal (of kings?)
I believe the answer is:
three
'a pair' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'royal of kings?' is the wordplay.
I cannot quite understand how this works, but
'royal' could be 'r' (abbreviation) and 'r' is located in the answer.
The remaining letters 'thee' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for three that I've seen before include "Prime number" , "A crowd, you might say" , "Feet in 3 Down" , "Number of Furies or Norns" , "Cardinal number" .)