Stop at sea beginning to assist Titanic (5)
I believe the answer is:
'stop at sea' is the definition.
(nautical term for 'stop')
'beginning to assist titanic' is the wordplay.
'beginning to' suggests taking the first letters.
'titanic' becomes 'vast' (both can mean very large).
The initial letter of 'assist' is 'a'.
(Other definitions for avast that I've seen before include "Stop (nautical)" , "Stop at sea" , "Stop sailing" , ""stop" signal" , "Stop! (naut.)" .)