Vital supplier cultivated vine (4)

I believe the answer is:

vein

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'vital supplier' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'cultivated vine' is the wordplay.
'cultivated' indicates an anagram.
'vine' anagrammed gives 'VEIN'.

Can you help me to learn more?

(Other definitions for vein that I've seen before include "Rock fissure or minerals therein" , "Distinctive quality" , "Leaf rib; ore layer in rock" , "Blood-vessel" , "Speaker's useless" .)

I've seen this clue in The Mirror.

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