Crossword Genius

Old sailor to return fire? (9)

Ross

I believe the answer is:

shellback

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

'old sailor' is the definition.
'shellback' can be an answer for 'sailor' (thesaurus). I'm unsure of the 'old' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.
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