One successful outcome of trade negotiations — perfect! (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'one successful outcome of trade negotiations' is the wordplay.
'one' becomes 'I' (Roman numeral).
'successful outcome of trade negotiations' becomes 'deal' (negotiations might result in a deal).
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "supreme" , "Model of perfection" , "Exemplar" , "Ultimate standard of perfection" , "superb specimen" .)