Cat - it's unusually still (6)
I believe the answer is:
static
'still' is the definition.
(static can mean still or not moving)
'cat it's unusually' is the wordplay.
'unusually' is an anagram indicator.
'cat'+'its'='catits'
'catits' with letters rearranged gives 'STATIC'.
(Other definitions for static that I've seen before include "Still - crackling on the radio" , "Still - radio interference" , "Unmoving, unchanging" , "Crackling or hissing noise caused by electrical interference" , "Constant interference" .)