A success, despite being below par (6)
I believe the answer is:
birdie
'a success' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'being below par' is the wordplay.
'being' becomes 'bird' (bird is a kind of being).
'below par' becomes 'ie' (I am not sure about this - if you are sure you should believe this answer much more).
'bird'+'ie'='BIRDIE'
'despite' is the link.
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" .)