Powerfulness or effectiveness (7)
I believe the answer is:
potency
I believe this is a double definition.
'powerfulness' is the first definition.
(potency is a kind of powerfulness)
'effectiveness' is the second definition.
(synonyms)
(Other definitions for potency that I've seen before include "Power; pet cony (anag.)" , "Strength - effectiveness" , "Might" , "authority" , "Capacity" .)