The answers to the flat below are homophones - they sound exactly the same but are spelled differently. The first answer word is a familiar one, but the second is not.

The taste tester's now been replaced
   with a bot.
When it senses a WHAT, it sets off a
   loud WATT.

Answer:

The taste tester's now been replaced 
   with a bot.
When it senses a TOXIN, it set off a
   loud TOCSIN.

Tocsin is an alarm bell or signal.

Credit:
The Enigma 
January, 2020
LEXMAN