Sets up secret organisation (6)
I believe the answer is:
Here is my best explanation:
'sets up' is the definition.
('erect' can be a synonym of 'set up')
'secret organisation' is the subsidiary indication.
'organisation' is an anagram indicator.
'secret' is an anagram of 'erects'.
(Other definitions for erects that I've seen before include "Puts up, builds", "Puts up, stand or building", "Builds, constructs", "Put up, builds", "Constructs, puts up", "Secret (anag.)", "Builds, puts up", "Sets up or builds." and "Puts up, say building".)