The Clue What's never free of charge?
The Answer ION
The answer "ION" fits the clue "What's never free of charge" because ions are charged particles. They can have a positive or negative charge, meaning they always have a "charge" associated with them. Since the clue plays on the word "charge," it cleverly refers to the scientific concept of ions. Thus, ions are never free of charge in both a literal and playful sense.