Conclusion when penalty crosses a line (6)
I believe the answer is:
finale
'conclusion' is the definition.
('finale' can be a synonym of 'conclusion')
'penalty crosses a line' is the wordplay.
'penalty' becomes 'fine' (fine is a kind of penalty).
'crosses' is an insertion indicator.
'line' becomes 'l'.
'a'+'l'='al'
'fine' going around 'al' is 'FINALE'.
'when' acts as a link.
(Other definitions for finale that I've seen before include "Conclusion of a piece of music" , "Denouement" , "Ending" , "Final section of a musical composition" , "Last song etc" .)