Power, vigour (8)
I believe the answer is:
strength
I believe this clue is a double definition.
'power' is the first definition.
(I know that strength is a type of power)
'vigour' is the second definition.
(I've seen this before)
(Other definitions for strength that I've seen before include "The longest one-syllable word in English - might" , "Brawn, muscle power." , "Force, might, potency" , "Force, firmness" , "Beneficial attribute" .)