### Clue: Treat for dog, with dog getting energy for nothing (4)
### Answer: CHEW 
 ### Explanation: Definition: Treat for dog
 "chow"(a sturdy Chinese breed of dog) **with** "e"(symbol for "energy" in physics) **replacing**(for) "o"(letter representing 0/nothing).
Defn: Something for a dog who, like all dogs, naturally loves to chew.