For Spooner, dubious material for policemen is something to chew on (7)
I believe the answer is:
popcorn
'chew on' is the definition.
The answer and definition can be both things that one consumes for nourishment as well as being singular nouns.
Maybe you can see an association between them that I don't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for popcorn that I've seen before include "Puffed maize" , "cinema fare" , "Maize heated into crisp edible balls, often eaten in cinema" , "modified maize" , "Kernels exploded by heat" .)