Oatmeal intermittently used by designer for such a picture (7)
I believe the answer is:
diorama
'such a picture' is the definition.
(diorama is a kind of picture)
'oatmeal intermittently used by designer' is the wordplay.
'intermittently' means one should take alternating letters.
'used by' means one lot of letters go next to another.
'designer' becomes 'dior' (dior is a designer brand).
The alternate letters of 'oatmeal' are 'ama'.
'ama' put after 'dior' is 'DIORAMA'.
'for' acts as a link.
(Other definitions for diorama that I've seen before include "Miniature setting" , "3-D model representing a scene" , "A little set" , "Miniature three- dimensional scene" , "AM radio? (anag)" .)