Number's up for boy (3)
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" .)