A massive sailor's cry (5)

I believe the answer is:

avast

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'sailor's cry' is the definition.
(nautical term meaning 'stop')

'a massive' is the wordplay.
'massive' becomes 'vast' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'vast'='AVAST'

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(Other definitions for avast that I've seen before include "Stop at sea" , "Hold fast! (naut.)" , "Stop, nautically speaking" , "order for tar" , "shout stopping the ship" .)

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