Stonefish? (4)
'stonefish?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for rock that I've seen before include "solid?" , "Large stone - type of music" , "Move side to side" , "Style of music; shock" , "Boulder - oscillate - sweetmeat" .)
(Other definitions for sardine that I've seen before include "Young pilchard" , "Swimmer" , "(Cramped?) fish" , "Fish (often tinned)" , "A diner's small fish" .)