Still acts strangely about half the time (6)
I believe the answer is:
static
'still' is the definition.
('static' can be a synonym of 'still')
'acts strangely about half the time' is the wordplay.
'strangely' is an anagram indicator.
'about' indicates putting letters inside.
'half' suggests halving the letters.
'acts' is an anagram of 'stac'.
'time' halved is 'ti'.
'stac' enclosing 'ti' is 'STATIC'.
(Other definitions for static that I've seen before include "stable" , "disturbing thing" , "Inert, motionless" , "Motionless like some kind of electricity" , "Not active, immobile" .)