Its subject: in messy warfare, male lost (1,8,2,4)
I believe the answer is:
a farewell to arms
'its subject' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'messy warfare male lost' is the wordplay.
'messy' indicates anagramming the letters.
'warfare'+'male'+'lost'='warfaremalelost'
'warfaremalelost' anagrammed gives 'A FAREWELL TO ARMS'.
'in' is the link.
Can you help me to learn more?
(Other definitions for a farewell to arms that I've seen before include "experience for the Venus de Milo?" , "what one handing in weapon might say" , "What Venus de Milo might have said" , "Novel by Hemingway" , "Book" .)