... not great shape (8)
I believe the answer is:
tetragon
'not great shape' is the definition.
'tetragon' can be an answer for 'shape' (tetragon is a kind of shape). I'm unsure of the 'not great' bit.
'not great shape' is the wordplay.
'shape' indicates an anagram.
'not'+'great'='notgreat'
'notgreat' is an anagram of 'TETRAGON'.
Can you help me to learn more?
(Other definitions for tetragon that I've seen before include "Four-sided plane figure" , "Shape" , "Plane figure with four angles and sides" .)