Understand component of a tree (4)
I believe the answer is:
twig
I believe this clue is a double definition.
'understand' is the first definition.
(I know that twig is a more specific form of the action apprehend)
'component of a tree' is the second definition.
(I know that twig is part of the tree)
(Other definitions for twig that I've seen before include "Slender woody shoot" , "A small branch - cop on?" , "growth of smaller branches" , "very little from box, perhaps" , "Eventually grasp" .)