Knock a score back (3)
I believe the answer is:
rap
'knock' is the definition.
(I know that knock can be written as rap)
'a score back' is the wordplay.
'a score' becomes 'par' (par is a kind of score).
'back' shows that the letters should be reversed in order.
'par' back-to-front is 'RAP'.
(Other definitions for rap that I've seen before include "Hit - "songs" by Snoop Dogg and Jay-Z, say" , "Form of music with chanted rhyming lyrics" , "Knock, tap" , "Knock loudly" , "Form of music with spoken lyrics" .)