Promise of power, large advantage (6)
I believe the answer is:
pledge
'promise' is the definition.
(I know that pledge is a type of promise)
'power large advantage' is the wordplay.
'power' becomes 'p' (symbol in physics).
'large' becomes 'L' (eg in clothes sizes).
'advantage' becomes 'edge' (having the edge is having an advantage).
'p'+'l'+'edge'='PLEDGE'
'of' acts as a link.
(Other definitions for pledge that I've seen before include "Plight" , "Promise - security" , "Promise, word of honour" , "Give one's word" , "assurance" .)