At last install a power circuit (3)
I believe the answer is:
lap
'circuit' is the definition.
(I know that circuit can be written as lap)
'at last install a power' is the wordplay.
'at last' says to take the final letters.
'power' becomes 'p' (symbol in physics).
The last letter of 'install' is 'l'.
'l'+'a'+'p'='LAP'
(Other definitions for lap that I've seen before include "Drink (like dog), circuit" , "Circuit of racetrack" , "place for small computer?" , "Use tongue to drink; circuit" , "One complete circuit" .)