Crossword Genius

Vital supplier upset a loiterer (9)

Ross

I believe the answer is:

arteriole

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'vital supplier' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'upset a loiterer' is the wordplay.
'upset' indicates an anagram.
'a'+'loiterer'='aloiterer'
'aloiterer' with letters rearranged gives 'ARTERIOLE'.

Can you help me to learn more?

(Another definition for arteriole that I've seen is " blood vessel".)

I've seen this clue in The Daily Express.
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