Clue: Weapon one with power to act on Royal house (5,6)
Answer: AGENT ORANGE
Explanation: Weapon one with power to act on Royal house (5,6)
**AGENT** ("one with power to act") *on* **ORANGE** ("Royal house") 
*Agent Orange is a herbicide, used to defoliate forests to so that enemy fighters can't use them for shelter, but its side effects cause illness to humans, so it could be considered a weapon.