Solo number coming up — do badly with notes (2,4,3)
I believe the answer is:
on ones tod
'solo' is the definition.
(I've seen this before)
'number coming up do badly with notes' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'coming up' shows that the letters should be reversed in order (in a down clue, letters go up).
'badly with' indicates anagramming the letters (letters in the wrong order).
'no' reversed gives 'on'.
'do' with its letters mixed up with 'notes' makes 'onestod'.
'on'+'onestod'='ON ONES TOD'
(Other definitions for on ones tod that I've seen before include "solo" , "reverse of 9 [AIDED] ?" , "Alone" .)