The most common and most likely answer to this crossword clue is the 4 plus 5 letter word VICE SQUAD
I've seen this crossword clue in The Guardian
V
I
C
E
S
Q
U
A
D
Buddy explains!
The straight part
We know that "Police who want use of crackdown/ say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case police who want use of crackdown/ say = vice squad.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.