Note low discount (4,4)
I believe the answer is:
mark down
'discount' is the definition.
(thesaurus)
'note low' is the wordplay.
'note' becomes 'mark' (synonyms).
'low' becomes 'down' (similar in meaning).
'mark'+'down'='MARK-DOWN'
(Other definitions for mark down that I've seen before include "Score points off" , "Lower cost" , "Reduce in price" .)