Still so rubbish inside (6)
I believe the answer is:
static
'still' is the definition.
(static can mean still or not moving)
'so rubbish inside' is the wordplay.
'so' becomes 'sic' ('so' in Latin).
'rubbish' becomes 'tat' (tat can mean rubbish or poor-quality material).
'inside' means one lot of letters goes inside another.
'sic' placed around 'tat' is 'STATIC'.
(Other definitions for static that I've seen before include "Inactive or inert" , "stable" , "Cast it (anag) - unmoving" , "Stuck in a traffic jam" , "Inactive, motionless" .)