Shipping line's carrying artillery to fight source of evil (8,3)
I believe the answer is:
pandoras box
'source of evil' is the definition.
(source of evil in Greek mythology)
'shipping line's carrying artillery to fight' is the wordplay.
'shipping line's' becomes 'pandos' (P and O is a cruise business).
'carrying' is an insertion indicator.
'artillery' becomes 'ra' (Royal Artillery).
'to fight' becomes 'box' (boxing is a kind of fighting).
'pandos' going around 'ra' is 'pandoras'.
'pandoras'+'box'='PANDORAS BOX'
(Other definitions for pandoras box that I've seen before include "legendary source of trouble" , "Mythical container of all the evils of the world" , "It was disastrous, for openers" , "Something that it's unwise to open?" , "In Greek mythology, what the first woman was forbidden to open" .)