Strictly correct about first solid geometrical shape (5)
I believe the answer is:
prism
'geometrical shape' is the definition.
'prism' can be an answer for 'shape' (prism is a kind of shape). I am unsure of the 'geometrical' bit.
'strictly correct about first solid' is the wordplay.
'strictly correct' becomes 'prim' (I've seen this before. I am not sure about the 'strictly' bit.).
'about' means one lot of letters goes inside another.
'first solid' becomes 's' (pst letter of 'solid').
'prim' going around 's' is 'PRISM'.
Can you help me to learn more?
(Other definitions for prism that I've seen before include "Female character in The Importance of Being Ernest -- optical device" , "Cut-glass object" , "Device that breaks light into colours of rainbow" , "Triangular shape of glass or quartz, breaks up a beam" , "Triangular piece of glass to deviate a beam" .)