Boy transgressed when upset (6)
I believe the answer is:
dennis
'boy' is the definition.
'transgressed when upset' is the wordplay.
'transgressed' becomes 'sinned' (sinning is a kind of transgressing).
'when upset' shows that the letters should be reversed in order (in a down clue - set, written, up).
'sinned' backwards is 'DENNIS'.
(Other definitions for dennis that I've seen before include "___ the Menace (in comic)" , "Man" , "-- Wheatley, English author" , "-- Potter, playwright" , "- Wheatley, English novelist" .)