The most common and most likely answer to this crossword clue is the 4 plus 5 letter word SPUR ROYAL
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The straight part
We know that "a bit of old gold" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a bit of old gold = spur royal.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 SPUR ROYAL mean?
SPUR ROYAL noun- A former English gold coin, first made in the reign of Edward IV, having a star on the reverse resembling the rowel of a spur.