The most common and most likely answer to this crossword clue is the 5 plus 3 plus 4 letter word BLACK AND TANS
I've seen this crossword clue in Financial Times
B
L
A
C
K
A
N
D
T
A
N
S
Buddy explains!
The straight part
We know that "Former police/Ds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case former police/ds = black and tans.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.