On one's toes (5)
I believe the answer is:
alert
'on one's toes' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for alert that I've seen before include "Warning signal" , "Signal danger" , "Watchful, sharp" , "'Attentive, vigilant (5)'" , "'Attentive, watchful (5)'" .)