Steve H,
Go on, then. If you mean the word "wire", which is irrelevant to the answer and only there to make the clue make more sense, I agree: it always irritated me a bit.
It’s worse than that. Here’s the clue again:
“She has copper wire, thin but very strong (9)"
It's weak because the perfect clue should make sense but also be as misleading as possible. Think of “Bar of soap (6,6)” for example. “Bar of soap” makes good sense in English ok, but it takes you away from the answer (“ROVER’S RETURN”).
In your clue the “copper wire” immediately gives away that it’s referring to the element copper, so the CU is obvious. Had it just been “She has copper, thin but very strong” (or better yet, “she meets copper, thin…etc” or similar) on the other hand there’d have been some ambiguity – ie, why has she met a policeman?
“She” and “her” aren’t interchangeable either by the way.