Agents detaining loony flipping in birthday suit (6)
I believe the answer is:
adamic
'in birthday suit' is the definition.
(I've seen this before)
'agents detaining loony flipping' is the wordplay.
'agents' becomes 'CIA' (Central Intelligence Agency).
'detaining' indicates putting letters inside.
'loony' becomes 'mad' (I've seen this in another clue).
'flipping' shows that the letters should be reversed in order.
'cia' placed around 'mad' is 'cimada'.
'cimada' in reverse letter order is 'ADAMIC'.
(Other definitions for adamic that I've seen before include "in birthday suit" , "Like the first man; naked" .)