Might (5)
'might' is the definition.
(being mighty)
This is all the clue.
(Other definitions for power that I've seen before include "Exponent" , "Supply with energy" , "Government" , "Supremacy, force" , "- - of attorney" .)
'might' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for strength that I've seen before include "A good quality; might" , "Physical energy" , "Fortitude" , "Physical or mental power" , "Brawn, muscle power." .)