Still it acts unusually (6)
I believe the answer is:
static
'still' is the definition.
(static can mean still or not moving)
'it acts unusually' is the wordplay.
'unusually' indicates an anagram.
'it'+'acts'='itacts'
'itacts' is an anagram of 'STATIC'.
(Other definitions for static that I've seen before include "Cast it (anag) - unmoving" , "Still - radio interference" , "Not in physical motion" , "Still, unchanging" , "it can shock and > ruin the reception" .)