Birds, half of them American (4)
I believe the answer is:
'birds' is the definition.
(emu is a kind of bird)
'half of them american' is the wordplay.
'half of' indicates taking half.
'american' becomes 'us' (abbreviation for United States).
'them' with half the letters taken is 'em'.
(Other definitions for emus that I've seen before include "Muse about flightless Australian birds" , "Running birds" , "Fast, but not high-fliers" , "laboratory units" , "Large flightless birds of Australia" .)