Tree knot (4)
(Another definition for burl that I've seen is " Small knot in wool".)
'tree knot' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gnarl that I've seen before include "Something lumpy" , "Rough lump (on a tree)" , "Lump, growl" , "lump of wood" , "Lump or knot in a tree" .)