Possible Clues:

• Toxic stuff
• Taken in a spoon, one will kill you
• One of this in a broken spoon could be the death of you (6)
• Eg, strychnine
• It causes illness or death in a living organism
• One in a broken spoon could be the very last thing you'd have
• Venom, harmful substance
• Seems the river's willing to be a killer
• That's the very last thing you'd take in a spoon
• Dangerous substance Petty Officer is taking regularly