Ruthless competition (3,3,3)
I believe the answer is:
dog eat dog
'ruthless competition' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dog eat dog that I've seen before include "Condition of savage self-interest" , "Ruthless (canine cannibalism?)" , "Savagely competitive" , "aggressive" , "fierce competition" .)