What does the cryptic crossword clue "Old iron — a material that's dead (8)" mean?

The definition is "dead". The wordplay uses an anagram. anagram of OLD + IRON + A = DOORNAIL; definition: "dead" The answer is DOORNAIL.

Old iron — a material that's dead (8)

Guardian Cryptic #24781 | 17 Across | 2009-08-18 flower