Antelope starts to get nervous, uneasy (3)
I believe the answer is:
gnu
'antelope' is the definition.
(gnu is a kind of antelope)
'starts to get nervous uneasy' is the wordplay.
'starts to' suggests taking the first letters.
The initial letters of 'get nervous uneasy' is 'GNU'.
(Other definitions for gnu that I've seen before include "Strange gun for an antelope" , "Animal" , "Wildebeest" , "Gun for a kind of deer" , "Literally wild beast" .)