Willingly change file (4)

I believe the answer is:

lief

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

'change file' is the wordplay.
'change' indicates an anagram.
'file' is an anagram of 'LIEF'.

(Another definition for lief that I've seen is " In old usage, gladly or willingly - file away".)

I've seen this clue in The Telegraph.

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