Knot in a tree (5)
I believe the answer is:
gnarl
'knot in a tree' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gnarl that I've seen before include "Rough lump (on a tree)" , "Something lumpy" , "lump of wood" , "Lump, growl" , "Lump or knot in a tree" .)