Pyramid model done rather crudely (11)
I believe the answer is:
tetrahedron
'pyramid' is the definition.
(I've seen this before)
'model done rather crudely' is the wordplay.
'model' becomes 'T' (Ford Model T car).
'crudely' indicates an anagram.
'done'+'rather'='donerather'
'donerather' is an anagram of 'etrahedron'.
't'+'etrahedron'='TETRAHEDRON'
(Other definitions for tetrahedron that I've seen before include "Four-faced solid" , "Solid with four triangular faces" , "Body with four triangular faces" , "Four-sided figure" , "Four-sided solid" .)