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