The most common and most likely answer to this crossword clue is the 7 letter word SPELTER
S
P
E
L
T
E
R
Buddy explains!

The straight part
We know that "Impure metal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case impure metal = spelter.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 SPELTER mean?
SPELTER noun- Zinc, often in blocks or ingot form.
- Zinc alloyed with another metal (especially copper), used as a solder.
- An objet d'art made from zinc.