Crossword Genius

It's a necessity to want to exist (4,2)

Ross

I believe the answer is:

need be

'it's a necessity' is the definition.
(I've seen this before)

'want to exist' is the wordplay.
'want' becomes 'need' (needing is a kind of wanting).
'to exist' becomes 'be' ('be' can be a synonym of 'exist').
'need'+'be'='NEED-BE'

'to' acts as a link.

I've seen this clue in The Globe and Mail.
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