Bending tree not hard, solid (11)
I believe the answer is:
tetrahedron
'solid' is the definition.
(tetrahedron is a kind of solid)
'bending tree not hard' is the wordplay.
'bending' indicates anagramming the letters.
'tree'+'not'+'hard'='treenothard'
'treenothard' is an anagram of 'TETRAHEDRON'.
(Other definitions for tetrahedron that I've seen before include "Four-sided solid" , "Body with four triangular faces" , "Four-faced solid" , "Solid with four triangular faces" , "Four-sided figure" .)