Crossword Genius

Hard tack (4)

Ross

I believe the answer is:

nail

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

'tack' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for nail that I've seen before include "Metal pin, hit it on the head" , "thing for keeping things together" , "It needs a hammer (or a file?)" , "Hammered individual" , "Metal fastener" .)

I've seen this clue in The Globe and Mail.
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