Aussie native married within European Union (3)
I believe the answer is:
emu
'aussie native' is the definition.
(bird native to Australia)
'married within european union' is the wordplay.
'married' becomes 'm'.
'within' means one lot of letters goes inside another.
'european union' becomes 'eu'.
'm' placed inside 'eu' is 'EMU'.
(Other definitions for emu that I've seen before include "Aussie bird" , "Large Australian flightless bird" , "Hull's character" , "Big bird" , "European project" .)