Nutcases operated within intelligence service (6)
I believe the answer is:
crania
'nutcases' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.
'operated within intelligence service' is the wordplay.
I cannot really see how this works, but
'operated' could be 'ran' ('run' can be a synonym of 'operate') and 'ran' is found within the answer.
'intelligence' could be 'cia' (Central Intelligence Agency) and 'cia' is found in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for crania that I've seen is " Skulls".)