Power in a simple electrical unit (6)
I believe the answer is:
ampere
'electrical unit' is the definition.
(I've seen this before)
'power in a simple' is the wordplay.
'power' becomes 'p'.
'in' indicates putting letters inside.
'simple' becomes 'mere' ('mere' can be a synonym of 'simple').
'a'+'mere'='amere'
'p' put into 'amere' is 'AMPERE'.
(Other definitions for ampere that I've seen before include "Current analyst" , "Current measure" , "Unit of electrical current" , "Solenoid inventor" , "Basic SI unit of electric current" .)