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 subsidiary indication.
'number's' becomes 'nos' (no. is a kind of number).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'nos' back-to-front is 'son'.
'for' is the link.
(Other definitions for son that I've seen before include "Offspring", "Heir", "Male descendant", "Native", "Male scion", "Young boy", "A relative", "Scion", "Male offspring" and "Child".)