The most common and most likely answer to this crossword clue is the 6 plus 2 plus 4 letter word BATTLE OF WITS
B
A
T
T
L
E
O
F
W
I
T
S
Buddy explains!
The straight part
We know that "“Holmes vs Moriarty”, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case “holmes vs moriarty”, say = battle of wits.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.