Against accepting defeat? One’s in the soup (7)
I believe the answer is:
crouton
'one's in the soup' is the definition.
(I've seen this before)
'against accepting defeat?' is the wordplay.
'against' becomes 'con' (as in pros and cons, for and against).
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'defeat?' becomes 'rout' (rout is a kind of defeat).
'con' placed around 'rout' is 'CROUTON'.
(Other definitions for crouton that I've seen before include "Bit of toast in soup" , "little bit of bread" , "Small cube of fried bread" , "Fried garnish for soup" , "lands in the soup" .)