Nixon was one to think endlessly about spies (8)
I believe the answer is:
magician
'nixon' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?
'one to think endlessly about spies' is the wordplay.
'one to think' becomes 'imagine' (I've seen this before).
'endlessly' means to look at the middle letters (the ends of the words are removed).
'about' is an insertion indicator.
'spies' becomes 'CIA' (Central Intelligence Agency).
The centre of 'imagine' is 'magin'.
'magin' going around 'cia' is 'MAGICIAN'.
'was' acts as a link.
Can you help me to learn more?
(Other definitions for magician that I've seen before include "Stage conjurer or maker of legerdemain" , "Person with a wand or rabbit in his hat?" , "Sorcerer, wizard" , "Conjuror, illusionist" , "Conjuror, wizard" .)