The most common and most likely answer to this crossword clue is the 3 plus 6 plus 6 letter word BOW STREET RUNNER
I've seen this crossword clue in The Daily Telegraph
B
O
W
S
T
R
E
E
T
R
U
N
N
E
R
More clues leading to the result BOW STREET RUNNER
Buddy explains!
The straight part
We know that "Old copper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old copper = bow street runner.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.