Confess little nonentity will shrug off defeat (6)
I believe the answer is:
squeak
'shrug off defeat' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'confess little nonentity will' is the wordplay.
'confess' becomes 'squeal' (synonyms).
'little' means to remove the last letter (the word is not quite complete).
'nonentity will' becomes 'k' (I can't justify this - if you can you should believe this answer much more).
'squeal' with its final letter removed is 'squea'.
'squea'+'k'='SQUEAK'
Can you help me to learn more?
(Other definitions for squeak that I've seen before include "Shrill cry - narrow escape" , "Bubble and --" , "Pipe" , "Pip's partner -- quakes (anag)" , "Short, high-pitched sound" .)