Crossword Genius

Army threat to sailors? (7)

Ross

I believe the answer is:

octopus

I'm a little stuck... Click here to teach me more about this clue!

'army threat' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'to sailors?' is the wordplay.
I cannot quite understand how this works, but
'to' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for octopus that I've seen before include "Spineless creature" , "Sea creature's tentacles served as food" , "Scoot up (anag.)" , "Eight-limbed cephalopod" , "army swimmer" .)

I've seen this clue in The Sun.
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