Second in command collecting junk still (6)
I believe the answer is:
static
'still' is the definition.
(static can mean still or not moving)
'second in command collecting junk' is the wordplay.
'second in command' becomes 'sic' (military abbreviation).
'collecting' indicates putting letters inside.
'junk' becomes 'tat' (both can mean rubbish).
'sic' placed around 'tat' is 'STATIC'.
(Other definitions for static that I've seen before include "Not moving or changing" , "Constant interference" , "Crackling or hissing noise caused by electrical interference" , "Changeless" , "Atmospheric disturbance" .)