Crossword Genius

Everything unknown to friend (4)

Ross

I believe the answer is:

ally

'to friend' is the definition.
(an ally is a friend or supporter)

'everything unknown' is the wordplay.
'everything' becomes 'all' (all things).
'unknown' becomes 'y' (algebra).
'all'+'y'='ALLY'

(Other definitions for ally that I've seen before include "Friend, partner" , "Associate that provides assistance" , "Team up" , "Friend and colleague" , "Friendly nation" .)

I've seen this clue in The Mirror.
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