Term of usefulness of a file? (4)

I believe the answer is:

life

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'term' is the definition.
(I know that life is a type of term)

'usefulness of a file?' is the wordplay.
'file' anagrams to 'LIFE'.
But, I am not sure how the anagram is indicated.

'of' is the link.

Can you help me to learn more?

(Other definitions for life that I've seen before include "The longest sentence" , ""But an empty dream" (Longfellow) - "As tedious as a twice-told tale" (Shakespeare)" , "Baffin oiler (anag) - 7 film" , "Animation" , "'While there's .... there's hope'" .)

I've seen this clue in the Evening Standard.

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