Number's up for boy (3)
I believe the answer is:
Here is my best explanation:
'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 "Offspring", "Native", "Young boy", "A relative", "Scion", "Descendant", "Male descendant", "Heir", "Male child" and "Boy".)