Colour popular with a fieldmouse (7)
I believe the answer is:
oatmeal
'colour' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for oatmeal that I've seen before include "Greyish-fawn colour flecked with brown" , "AA Motel (anag.)" , "It's boiled for porridge" , "Kind of porridge" , "Ground grain used to make porridge" .)