The most common and most likely answer to this crossword clue is the 6 letter word DETECT
I've seen this crossword clue in The Daily Telegraph
D
E
T
E
C
T
Buddy explains!
The straight part
We know that "Principally, discover exactly the evidence coppers target" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case principally, discover exactly the evidence coppers target = detect.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 DETECT mean?
DETECT verb- To discover or find by careful search, examination, or probing
- Detected.