Secret that is about new heads of state in Denmark (6)
I believe the answer is:
inside
'secret' is the definition.
('inside' can be a synonym of 'secret')
'that is about new heads of state in denmark' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'about' is an insertion indicator.
'heads of' suggests taking the first letters.
'state in denmark' becomes 'sid' (this might be a standard abbreviation which I don't know about).
The first letter of 'new' is 'n'.
'n'+'sid'='nsid'
'ie' placed around 'nsid' is 'INSIDE'.
Can you help me to learn more?
(Other definitions for inside that I've seen before include "Behind bars (sl.)" , "Middle" , "serving porridge" , "(Of information) from a confidential source" , "Within - interior" .)