Office machine that is right behind policeman (6)
I believe the answer is:
copier
'office machine' is the definition.
(I've seen this before)
'that is right behind policeman' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'right' becomes 'r' (common abbreviation).
'behind' means one lot of letters go next to another.
'policeman' becomes 'cop' (slang term).
'ie'+'r'='ier'
'ier' put after 'cop' is 'COPIER'.
(Other definitions for copier that I've seen before include "piece of office equipment" , "Office machine" , "Something that duplicates" , "Reproducer" , "Machine to reproduce print" .)