Leak aim about containing Brussels complaint (9)
I believe the answer is:
leukaemia
'complaint' is the definition.
(leukaemia is a kind of health problem)
'leak aim about containing brussels' is the wordplay.
'about' indicates an anagram.
'containing' indicates putting letters inside.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
'leak'+'aim'='leakaim'
'leakaim' with letters rearranged gives 'lkaemia'.
'lkaemia' enclosing 'eu' is 'LEUKAEMIA'.
(Other definitions for leukaemia that I've seen before include "Disease in which white blood cells multiply" , "Type of cancer" , "Cancer of the blood" .)