Heartlessly let two into game, then told them they're too old (3,5)
I believe the answer is:
age limit
'old' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I don't see?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for age limit that I've seen before include "terminus a quo, say" , "I am legit (anag)" , "Regulation based on year of birth that sets down when one can no longer do something" , "Bar to those too old or too young" , "Maturity restriction" .)