At the outset innovative agreement is perfect (5)
I believe the answer is:
ideal
'is perfect' is the definition.
(similar in meaning)
'at the outset innovative agreement' is the wordplay.
'at the outset' says to take the initial letters (the outset is the beginning).
'agreement' becomes 'deal' (deal is a kind of agreement).
The first letter of 'innovative' is 'i'.
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "Quite perfect, can't be improved on" , "Best possible" , "Coal-mining waste" , "without flaws" , "example" .)