Tunny (4)
I believe the answer is:
tuna
'tunny' is the definition.
(I know that tunny can be written as tuna)
This is the entire clue.
(Other definitions for tuna that I've seen before include "Type of fish" , "Warm-water food fish" , "Species of fish" , "Fish; edible cactus" , "Widely-eaten fish" .)