Note power circuit (3)
I believe the answer is:
lap
'circuit' is the definition.
(I know that circuit can be written as lap)
'note power' is the wordplay.
'note' becomes 'la' (musical note as in do-re-mi).
'power' becomes 'p' (symbol in physics).
'la'+'p'='LAP'
(Other definitions for lap that I've seen before include "Single circuit of track" , "Circuit of racetrack" , "400 metres, typically" , "Once around the track" , "Track circuit; drink" .)