Australian runner getting married in Brussels (3)
I believe the answer is:
emu
'australian runner' is the definition.
(bird which runs)
'married in brussels' is the wordplay.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'in' indicates putting letters inside.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
'm' inserted within 'eu' is 'EMU'.
'getting' acts as a link.
(Other definitions for emu that I've seen before include "Australian bird related to the ostrich" , "that doesn't get off the ground" , "Ratite" , "Fast runner in Oz" , "European project" .)