Three-dimensional scene Ivor regularly included in play (7)
I believe the answer is:
diorama
'three-dimensional scene' is the definition.
(a 3D model)
'ivor regularly included in play' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'included in' indicates putting letters inside.
'play' becomes 'drama' (synonyms).
The alternating letters of 'ivor' are 'io'.
'io' placed into 'drama' is 'DIORAMA'.
(Other definitions for diorama that I've seen before include "Model with 3-D figures" , "Scene with models" , "A little set" , "contrived scene" , "Miniature three- dimensional scene" .)