Trace bird from down under over time (4)
I believe the answer is:
atom
'trace' is the definition.
(thesaurus)
'bird from down under over time' is the wordplay.
'bird' becomes 'moa' (extinct flightless bird of New Zealand).
'from down under' says the letters should be written backwards (in a down clue, letters written from bottom to top).
'over' is an insertion indicator.
'time' becomes 't'.
'moa' back-to-front is 'aom'.
'aom' going around 't' is 'ATOM'.
(Other definitions for atom that I've seen before include "Scrap" , "A mighty particle, can be explosive" , "Basic unit of a chemical element" , "Very small bit" , "Particle of matter now divided" .)