Crossword Genius

One is grasping but supportive (7)

Ross

I believe the answer is:

tendril

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

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for tendril that I've seen before include "Threadlike extension of a plant" , "A creeper, or a curl of hair" , "Stem by which climbing plant attaches itself" , "Plant's climbing aid" , "Climbing plant's grasper" .)

I've seen this clue in the Evening Standard.
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