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 don't understand how one could define the 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 backwards.
'tuna' with its last letter removed is 'tun'.
'tun' in reverse letter order is '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" .)