The most common and most likely answer to this crossword clue is the 6 letter word OSMIUM
I've seen this crossword clue in The Times
O
S
M
I
U
M
More clues leading to the result OSMIUM
- Metal rings act, traditionally, as protection for one
- I’m very upset with university’s masculine element
- Heavy Metal single covered by Big Mama
- New mood music — missing old CD with heavy metal?
- Expand very large element
- Maker of nibs is outsize and quiet about one
- I am kept up by sumo wrestling and heavy metal
- Element of independence found in well-brought-up mother
- Element iodine injecting mother of Oscar?
- Very backward old woman accepts iodine for a metal
Buddy explains!

The straight part
We know that "Metal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case metal = osmium.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 OSMIUM mean?
OSMIUM noun- A chemical element (symbol Os) with atomic number 76: a hard, brittle, heavy, bluish-white transition metal found as a trace element in alloys, mostly in platinum ores.
- A single atom of this element.