Curious child, but likely to succeed? (4,2)
I believe the answer is:
odds on
'likely to succeed?' is the definition.
(betting term meaning likely to happen)
'curious child' is the wordplay.
'curious' becomes 'odd' (curious can mean odd or peculiar).
'child' becomes 'son' (son is a kind of child).
'odd'+'son'='ODDS-ON'
'but' acts as a link.
(Other definitions for odds on that I've seen before include "more or less given?" , "very much a favourite" , "betting price?" , "High probability" , "likely to succeed" .)