My rapid changes, solid (7)
I believe the answer is:
pyramid
'solid' is the definition.
(type of geometric solid)
'my rapid changes' is the wordplay.
'changes' is an anagram indicator.
'my'+'rapid'='myrapid'
'myrapid' with letters rearranged gives 'PYRAMID'.
(Other definitions for pyramid that I've seen before include "Four sided monument" , "Ancient Egyptian building" , "Egyptian stone structure" , "Type of polyhedron" , "type of selling" .)