Signal from squealer transmitted (5)
I believe the answer is:
alert
'signal' is the definition.
(alert is a kind of signal)
'from squealer transmitted' is the wordplay.
'from' indicates the answer is hidden within the clue.
'ALERT' is hidden within 'squealer transmitted'.
(Other definitions for alert that I've seen before include "Watchful and bright" , "Watchful, quick-thinking" , "'Attentive, vigilant (5)'" , "Warning signal" , "Prompt" .)