Physical type of knowledge in passage about Republic (6)
I believe the answer is:
carnal
'physical' is the definition.
(similar in meaning)
'in passage about republic' is the wordplay.
'in' indicates putting letters inside.
'passage' becomes 'canal' (canal is a kind of passage).
'about republic' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'canal' enclosing 'r' is 'CARNAL'.
'type of knowledge' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for carnal that I've seen before include "Relating to physical needs" , "Animal" , "Of the flesh, not spiritual" , "Earthly" , "Fleshly, sensual" .)