Youngster absorbing one round number — who benefits? (3,4)
I believe the answer is:
cui bono
'who benefits?' is the definition.
(I've seen this before)
'youngster absorbing one round number' is the wordplay.
'youngster' becomes 'cub' (I've seen this in other clues).
'absorbing' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'round number' becomes 'ono' (I can't justify this - if you can you should believe this answer much more).
'cub' enclosing 'i' is 'cuib'.
'cuib'+'ono'='CUI BONO'
(Other definitions for cui bono that I've seen before include "who gains?" , "Who gets any benefit from" , "Who benefits?" .)