Vital supplier cultivated vine (4)
I believe the answer is:
vein
'vital supplier' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'cultivated vine' is the wordplay.
'cultivated' is an anagram indicator.
'vine' is an anagram of '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" .)