Hard tack (4)
I believe the answer is:
nail
'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" .)