A success, despite being below par (6)
I believe the answer is:
birdie
'par' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe there's a link between them I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for birdie that I've seen before include "In golf, a score of one under par for the hole" , "One under par - watch it!" , "Good hole" , "Golfing score better than par" , "One under in 20 across" .)