Greatest power invested in certain author (7)
I believe the answer is:
supreme
'greatest' is the definition.
(I've seen this before)
'power invested in certain author' is the wordplay.
'power' becomes 'p'.
'invested in' indicates putting letters inside.
'certain' becomes 'sure' (synonyms).
'author' becomes 'me' (the author of the crossword).
'p' put inside 'sure' is 'supre'.
'supre'+'me'='SUPREME'
(Other definitions for supreme that I've seen before include "Ranked highest" , "Greatest in status, authority or power" , "Creamy sauce" , "God" , "Highest in authority; rich cream sauce (dish)" .)