The keys to knowledge are nature and time, he thought (4)
I believe the answer is:
book
'the keys to knowledge are nature and time he thought' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'he thought' is the wordplay.
'he' becomes 'Bo'.
'thought' becomes 'ok' (I can't explain this - if you can you should give a lot more credence to this answer).
'bo'+'ok'='BOOK'
Can you help me to learn more?
(Other definitions for book that I've seen before include "Reading volume" , "Quick work" , "Make a reservation" , "Take down name of offending player" , "Rebecca or Emma maybe" .)