Document written in legal language, principally (4)

I believe the answer is:

will

'document' is the definition.
(will is a kind of document)

'written in legal language principally' is the wordplay.
'principally' suggests taking the first letters.
The initial letters of 'written in legal language' is 'WILL'.

(Other definitions for will that I've seen before include "Intention, desire" , "Resolve - testament" , "Inclination" , "Fixed intention" , "self-control" .)

I've seen this clue in The Times.

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