Boy of about ten (3)
I believe the answer is:
rex
'boy' is the definition.
'about ten' is the wordplay.
'about' becomes 're' (regarding).
'ten' becomes 'x' (roman numeral).
're'+'x'='REX'
'of' acts as a link.
(Other definitions for rex that I've seen before include "One in the palace" , "Dog's name; king" , "Part of official title of a king" , "King - Harrison?" , "Oedipus - -" .)