A big hit when young (3)
I believe the answer is:
six
'a big hit' is the definition.
(cricket)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for six that I've seen before include "Number of bowls in a cricket over" , "Knock for . . . - surprise or defeat" , "Maximum cricket hit" , "Half-dozen" , "Airborne boundary hit" .)