Gnarled (6)
I believe the answer is:
knotty
'gnarled' is the definition.
(I know that gnarled can be written as knotty)
This is all the clue.
(Other definitions for knotty that I've seen before include "Tangled - snarled" , "Tree trunk with hard masses - it's problematic" , "Convoluted" , "Difficult to unravel" , "Full of tangles; extremely intricate" .)