Idiot going round south-east in secret (6)
I believe the answer is:
closet
'secret' is the definition.
('closet' can be a synonym of 'secret')
'idiot going round south-east in' is the wordplay.
'idiot' becomes 'clot' (I've seen this before**).
'going round' is an insertion indicator.
'south' becomes 's' ('S' can be a synonym of 'south').
'east' becomes 'e' ('E' can be a synonym of 'east').
'in' is an insertion indicator.
'clot' placed around 's' is 'clost'.
'clost' placed around 'e' is 'CLOSET'.
(Other definitions for closet that I've seen before include "Wardrobe in the US" , "Small room or cabinet for storage" , "Compartment for clothing in the US" , "Covert" , "Small room or wardrobe" .)