Knock up a short paragraph (3)
I believe the answer is:
par
'a short paragraph' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'knock up' is the wordplay.
'knock' becomes 'rap' (synonyms).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'rap' back-to-front is 'PAR'.
Can you help me to learn more?
(Other definitions for par that I've seen before include "standard for a pro" , "Accepted standard" , "Accepted level" , "Standard number of strokes for a golf-hole" , "Rating of strokes required for hole (golf)" .)