Boy, unknown before prison sentence, finding beetle (8)
I believe the answer is:
ladybird
'beetle' is the definition.
(I know that ladybird is a type of beetle)
'boy unknown before prison sentence' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'unknown' becomes 'y' (algebra).
'before' is a charade indicator (letters next to each other).
'prison sentence' becomes 'bird' (to 'do bird' is to serve a prison sentence).
'lad'+'y'+'bird'='LADYBIRD'
'finding' acts as a link.
(Other definitions for ladybird that I've seen before include "spotted one that flies" , "Kind of beetle" , "spotted crawler" , "Common insect" , "Red spotted beetle" .)