The most common and most likely answer to this crossword clue is the 6 plus 6 letter word DOCTOR WATSON
D
O
C
T
O
R
W
A
T
S
O
N
More clues leading to the result DOCTOR WATSON
Buddy explains!

The straight part
We know that "Detective’s assistant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case detective’s assistant = doctor watson.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.