The one beyond a pair (5)
I believe the answer is:
third
'pair' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Maybe there's an association between them I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for third that I've seen before include "Ruthless interrogation" , "Ordinal number" , "Winning bronze medal" , "Last of three" , "Musical interval" .)