Signal from whaler transmitted (5)
I believe the answer is:
alert
'signal' is the definition.
(I know that alert is a type of signal)
'from whaler transmitted' is the wordplay.
'from' indicates a hidden word.
'ALERT' is hidden in the letters of 'whaler transmitted'.
(Other definitions for alert that I've seen before include "Wakeful, observant" , "Signal danger" , "In state of watchfulness" , "Awake, attentive" , "On the ball" .)