It is sent back in warring couple's application (8)
I believe the answer is:
poultice
'application' is the definition.
(I've seen this before)
'it is sent back in warring couple's' is the wordplay.
'is sent back' says the letters should be written in reverse.
'in' indicates putting letters inside.
'warring' indicates anagramming the letters.
'it' written backwards gives 'ti'.
'couple' with letters rearranged gives 'poulce'.
'ti' inserted within 'poulce' is 'POULTICE'.
(Other definitions for poultice that I've seen before include "Hot medical dressing" , "one's meant to soothe" , "Couple it oddly with a medical dressing" , "Soft heated mass applied to inflamed area of skin" , "Heated medical application for drawing pus" .)