Birds, half of them American (4)
I believe the answer is:
emus
'birds' is the definition.
(emu is a kind of bird)
'half of them american' is the wordplay.
'half of' means to take half the letters.
'american' becomes 'US' (United States).
'them' cut in half is 'em'.
'em'+'us'='EMUS'
(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" .)