The most common and most likely answer to this crossword clue is the 6 letter word LATTEN
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The straight part
We know that "Old brass" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old brass = latten.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 LATTEN mean?
LATTEN noun- An alloy of copper and tin, similar to bronze, with a sufficient portion of tin to make it a pewter-like color with yellowish tinge (rather than the brownish-gold color of bronze of higher copper content), once used in thin sheets and for domestic utensils and light-duty tools.
- Sheet tin; iron plate, covered with tin; also, any metal in thin sheets.