Crossword Genius

What can make salt last? (7)

Ross

I believe the answer is:

anagram

'last?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand how the rest of the clue works.

(Other definitions for anagram that I've seen before include "Word play" , "Type of word puzzle" , "Jumbling of letters" , "'Master' of 'stream', say" , "There's confusion among characters" .)

I've seen this clue in The Times.
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