Set of principles and so on — hard one to get installed (5)
I believe the answer is:
ethic
'set of principles' is the definition.
(I've seen this before)
'and so on hard one to get installed' is the wordplay.
'and so on' becomes 'etc' (synonyms).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'one to get' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'installed' is an insertion indicator.
'h'+'i'='hi'
'etc' enclosing 'hi' is 'ETHIC'.
Can you help me to learn more?
(Other definitions for ethic that I've seen before include "Moral system" , "Moral code" , "Set of moral values" , "type of behaviour" , "Moral value" .)