Cook tuna mostly over ring (5)
I believe the answer is:
donut
'ring' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'cook tuna mostly over' is the wordplay.
'cook' becomes 'do' (do can mean to cook).
'mostly' means to remove the last letter (most but not all of the word).
'over' says the letters should be written in reverse.
'tuna' with its final letter taken away is 'tun'.
'tun' reversed gives 'nut'.
'do'+'nut'='DONUT'
Can you help me to learn more?
(Other definitions for donut that I've seen before include "something to eat" , "American treat?" , "American pastry" , "ring coming across the Atlantic" , "Sugary deep-fried cake" .)