A good round knock-up (3)
I believe the answer is:
par
'a good' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'knock-up' is the wordplay.
'knock' becomes 'rap' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'rap' reversed gives 'PAR'.
'round' is the link.
Can you help me to learn more?
(Other definitions for par that I've seen before include "State of equality" , "Accepted standard" , "Black music" , "Standard amount" , "Normal number of strokes for a hole at golf" .)