A brilliant discovery! Brussels are amazingly full of potassium (6)
I believe the answer is:
eureka
'a brilliant discovery' is the definition.
(I've seen this before)
'brussels are amazingly full of potassium' is the wordplay.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
'amazingly' indicates anagramming the letters.
'full of' means one lot of letters goes inside another.
'potassium' becomes 'K' (K is the chemical symbol for potassium).
'are' with letters rearranged gives 'rea'.
'rea' enclosing 'k' is 'reka'.
'eu'+'reka'='EUREKA'
(Other definitions for eureka that I've seen before include ""I've discovered it!"" , "Triumphant cry of Greek" , "Exclamation of triump from Archimedes" , "I've got it!" , "Cry of joy on finding something" .)