The usual knock-up (3)
I believe the answer is:
par
'the usual' is the definition.
(par for the course)
'knock-up' is the wordplay.
'knock' becomes 'rap' (synonyms).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'rap' reversed gives 'PAR'.
(Other definitions for par that I've seen before include "Median" , "Usual score at golf" , "Average - equality" , "Accepted standard" , "Normal golfing score" .)