Signal some whaler transmitted (5)
I believe the answer is:
alert
'signal' is the definition.
(I know that alert is a type of signal)
'some whaler transmitted' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'ALERT' is hidden in the letters of 'whaler transmitted'.
(Other definitions for alert that I've seen before include "Wakeful, observant" , "Prompt" , "Vigilant, watchful" , "Wide awake and watchful" , "Watchfully awake" .)