Gate to vine turned out a vital supply line (6,4)

I believe the answer is:

portal vein

I'm a little stuck... Click here to teach me more about this clue!

'out a vital supply line' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'gate to vine turned' is the wordplay.
'gate' becomes 'portal' (I've seen this in another clue).
'to' is a charade indicator (letters next to each other).
'turned' is an anagram indicator.
'vine' anagrammed gives 'vein'.
'portal'+'vein'='PORTAL VEIN'

Can you help me to learn more?

(Another definition for portal vein that I've seen is " What supplies liver".)

I've seen this clue in The Daily Express.

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