The most common and most likely answer to this crossword clue is the 5 letter word WILL*
I've seen this crossword clue in The Guardian
W
I
L
L
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Buddy explains!
The straight part
We know that "It comprises primarily what is left, legally" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it comprises primarily what is left, legally = will*.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does WILL* mean?
WILL* verb- (now uncommon or literary) To wish, desire (something).
- (nowadays rare) To wish or desire (that something happen); to intend (that).
- (auxiliary) To habitually do (a given action).
- (auxiliary) To choose to (do something); used to express intention but without any temporal connotations (+ bare infinitive), often in negation.
- (auxiliary) Used to express the future tense, sometimes with some implication of volition when used in the first person. Compare shall.
- (auxiliary) To be able to, to have the capacity to.
- (auxiliary) Expressing a present tense with some conditional or subjective weakening: "will turn out to", "must by inference".
- One's independent faculty of choice; the ability to be able to exercise one's choice or intention.
- One's intention or decision; someone's orders or commands.
- The act of choosing to do something; a person’s conscious intent or volition.
- Law A formal declaration of one's intent concerning the disposal of one's property and holdings after death; the legal document stating such wishes.
- That which is desired; one's wish.
- Desire, longing. (Now generally merged with later senses.)
- To wish, desire.
- To instruct (that something be done) in one's will.
- To try to make (something) happen by using one's will (intention).
- To bequeath (something) to someone in one's will (legal document).