Bird's unusual motion, edging at first upwards (3)
I believe the answer is:
emu
'bird's' is the definition.
(emu is a type of bird)
'unusual motion edging at first upwards' is the wordplay.
'at first' suggests taking the first letters.
'upwards' is a reversal indicator.
The first letters of 'unusual motion edging' is 'ume'.
'ume' reversed gives 'EMU'.
(Other definitions for emu that I've seen before include "European project" , "that doesn't get off the ground" , "Aussie runner" , "Ratite" , "Aussie bird" .)