A tremendous shout from the sailor (5)
I believe the answer is:
avast
'a tremendous shout from the sailor' is the definition.
I can't tell whether this definition defines the answer.
'a tremendous shout from the' is the wordplay.
'a tremendous shout' becomes 'avas' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'from' is a charade indicator (letters next to each other).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'avas'+'t'='AVAST'
Can you help me to learn more?
(Other definitions for avast that I've seen before include "Hold fast! (naut.)" , "Stop, nautically speaking" , "eleven [CEASE] at sea" , "Stop on board" , "Stop (nautical)" .)