Sea Denizen can perform on track (4,4)
I believe the answer is:
john dory
'sea denizen' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'can perform on track' is the wordplay.
'can' becomes 'john' (slang for toilet).
'perform' becomes 'do' (both can mean complete an action).
'on' is a charade indicator (letters next to each other).
'track' becomes 'ry' (abbreviation for railway).
'john'+'do'+'ry'='JOHN DORY'
Can you help me to learn more?
(Other definitions for john dory that I've seen before include "Tasty flatfish with black spot on sides" , "Edible fish" , "a catch!" , "Zeus" , "European fish with large jaws" .)