Stop at sea initially alerted by Titanic (5)
I believe the answer is:
avast
'stop at sea' is the definition.
(nautical term for 'stop')
'initially alerted by titanic' is the wordplay.
'initially' indicates taking the first letters.
'by' says to put letters next to each other.
'titanic' becomes 'vast' (both can mean very large).
The first letter of 'alerted' is 'a'.
'a'+'vast'='AVAST'
(Other definitions for avast that I've seen before include "order for tar" , "Stop on board" , "Stop, nautically speaking" , "eleven [CEASE] at sea" , "Stop sailing" .)