The up-and-down principle (5)
I believe the answer is:
tenet
'principle' is the definition.
(a tenet is a principle or value)
'the up-and-down' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'and-down' becomes 'tene' (I can't justify this - if you can you should believe this answer much more).
'tene' in reverse letter order is 'enet'.
't'+'enet'='TENET'
Can you help me to learn more?
(Other definitions for tenet that I've seen before include "Doctrine held to be true" , "That's the law" , "Fundamental belief" , "Palindromic principle or belief" , "Article of belief" .)