Knock up to standard (3)
I believe the answer is:
par
'standard' is the definition.
(thesaurus)
'knock up' is the wordplay.
'knock' becomes 'rap' ('rap' can be a synonym of 'knock').
'up' is a reversal indicator (in down clue: letters go upwards).
'rap' written backwards gives 'PAR'.
'to' is the link.
(Other definitions for par that I've seen before include "Normal level or standard" , "Standard score for golfers" , "Average score for round of golf" , "What is normal - . . . for the course" , "average degree" .)