Panic about start of real Brussels outburst (5-2)
I believe the answer is:
flare-up
'outburst' is the definition.
(I know that outburst can be written as flare-up)
'panic about start of real brussels' is the wordplay.
'panic' becomes 'flap' (getting in a flap is panicking).
'about' is an insertion indicator.
'start of' suggests taking the first letters.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
The first letter of 'real' is 'r'.
'r'+'eu'='reu'
'flap' going around 'reu' is 'FLARE-UP'.
(Other definitions for flare-up that I've seen before include "Suddenly get angry" , "Sudden outbreak (of violence, etc)" , "Burst into flames" , "Suddenly burn brightly" , "Burst into flame or become suddenly angry" .)