Detective on dream branch (8)
I believe the answer is:
division
'branch' is the definition.
(thesaurus)
'detective on dream' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'on' means one lot of letters go next to another.
'dream' becomes 'vision' (I've seen this before).
'di'+'vision'='DIVISION'
(Other definitions for division that I've seen before include "some police?" , "A number of teams grouped together in sport according to ability" , "Major unit of an organisation or maths function" , "Large army unit" , "Section of organisation" .)