Alert, notes noose loose (2,4,4)
I believe the answer is:
on ones toes
'alert' is the definition.
(I've seen this before)
'notes noose loose' is the wordplay.
'loose' indicates anagramming the letters (letters are loosened from their original positions).
'notes'+'noose'='notesnoose'
'notesnoose' anagrammed gives 'ON ONES TOES'.
(Other definitions for on ones toes that I've seen before include "ready" , "Alert; where the interfering tread" .)