One is grasping but supportive (7)
I believe the answer is:
tendril
'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" .)