Possible Clues:

• What's clearly smelt with head cut off, or another fish
• Awful smell, head cut off fish
• Foul smell left by small fish
• Fish stink when head is removed
• Carp variety
• Carp relative
• Fish fingers all cold, then hot
• Fish in ditch after river's drained
• Fish producing bad smell when head's cut off
• Fish — there's unknown number in school?