The most common and most likely answer to this crossword clue is the 3 plus 3 plus 3 letter word DOG EAT DOG
I've seen this crossword clue in The Daily Telegraph
D
O
G
E
A
T
D
O
G
Buddy explains!
The straight part
We know that "Ruthless competition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ruthless competition = dog eat dog.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DOG EAT DOG mean?
DOG EAT DOG adjective- Ruthlessly acquisitive or competitive. Describes a business or other set of circumstances where people try to succeed at the expense of other people.