Crossword Genius

Number's up for boy (3)

Ross

I believe the answer is:

son

'boy' is the definition.
('son' can be a synonym of 'boy')

'number's up' is the wordplay.
'number's' becomes 'nos' (abbreviation for 'number').
'up' is a reversal indicator (in down clue: letters go upwards).
'nos' backwards is 'SON'.

'for' is the link.

(Other definitions for son that I've seen before include "Human offspring" , "Heir" , "Male descendant" , "Native" , "Express paper" .)

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