Efficacy (7)

I believe the answer is:

potency

(Other definitions for potency that I've seen before include "Power; pet cony (anag.)" , "Might" , "Great power" , "Strength - effectiveness" , "authority" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.