Fish heads from Cornwall or Devon (3)
I believe the answer is:
cod
'fish' is the definition.
(type of fish)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cod that I've seen before include "to be paid for when it arrives" , "No payment in advance" , "Hoax (sl.); fish" , "A joke of a fish?" , "A foolish sort of a fish" .)