Bird kept by them, usually (3)
I believe the answer is:
emu
'bird' is the definition.
(emu is a kind of bird)
'kept by them usually' is the wordplay.
'kept by' indicates a hidden word.
'EMU' is hidden within 'them usually'.
(Other definitions for emu that I've seen before include "it can't get off the ground" , "unit of measurement" , "Flightless Aus. bird" , "one offering challenge to Hull?" , "Flightless bird of Australia, like ostrich" .)