Magistrate at party good in cut-throat competition (3-3-3)
I believe the answer is:
dog-eat-dog
'competition' is the definition.
(I have seen 'ruthless competition' mean 'dog eat dog' so perhaps 'competition' could also mean 'dog eat dog')
'magistrate at party good in cut-throat' is the wordplay.
I cannot quite see how this works, but
'magistrate' could be 'doge' (doge is a kind of magistrate) and 'doge' is present in the answer.
'at' is within the answer.
'party' could be 'do' (do is a kind of party) and 'do' is present in the answer.
'good' could be 'g' (abbreviation) and 'g' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for dog-eat-dog that I've seen before include "Ruthless competition as by cannibalistic canines" , "Savagely ruthless" , "Ruthless competition of canine cannibals" , "Ruthless competition - savage self-concern" , "Ruthless pursuit of one's own interests" .)