Spies block computer access, one reasons (8)
I believe the answer is:
logician
'access one reasons' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'spies block computer' is the wordplay.
'spies' becomes 'CIA' (Central Intelligence Agency).
'block' is an insertion indicator (upper walls off glass inside formal).
'computer' becomes 'login' (I can't explain this - if you can you should believe this answer much more).
'cia' going into 'login' is 'LOGICIAN'.
Can you help me to learn more?
(Other definitions for logician that I've seen before include "Expert in reasoning" , "Reasoned thinker" , "Rigorous reasoner" , "Expert reasoner" , "One who clearly thinks" .)