Detective's dream department (8)
I believe the answer is:
division
'department' is the definition.
(thesaurus)
'detective's dream' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'dream' becomes 'vision' (I've seen this before).
'di'+'vision'='DIVISION'
(Other definitions for division that I've seen before include "Large army unit" , "Boundary" , "Parliamentary vote" , "Section; maths process" , "Discord, split" .)