Insert another cartridge left over in study (6)
I believe the answer is:
reload
'insert another cartridge' is the definition.
(reloading a gun cartridge)
'left over in study' is the wordplay.
'left' becomes 'l' (common abbreviation).
'over' becomes 'o' (cricket abbreviation).
'in' is an insertion indicator.
'study' becomes 'read' (synonyms).
'l'+'o'='lo'
'lo' placed within 'read' is 'RELOAD'.
(Other definitions for reload that I've seen before include "Give further ammunition to" , "Put in another charge" , "Put bullet in gun - again" , "pack, once more" , "Put more bullets in" .)