Lots of paper bags left department (5)
I believe the answer is:
realm
'department' is the definition.
(I've seen this before)
'lots of paper bags left' is the wordplay.
'lots of paper' becomes 'ream' (I've seen this before).
'bags' means one lot of letters goes inside another (bag can mean secures or steals).
'left' becomes 'l' (common abbreviation).
'ream' enclosing 'l' is 'REALM'.
(Other definitions for realm that I've seen before include "field of study" , "Land under a king or queen" , "department" , "the UK, perhaps" , "monarch's responsibility" .)