Welcome cuts and so forth, in principle (5)
I believe the answer is:
ethic
'principle' is the definition.
(ethic is a kind of principle)
'welcome cuts and so forth' is the wordplay.
'welcome' becomes 'hi' (I've seen this in other clues).
'cuts' indicates putting letters inside (some letters cut their way into other letters).
'and so forth' becomes 'etc' ('etc.' can be a synonym of 'and so forth').
'hi' put inside 'etc' is 'ETHIC'.
'in' acts as a link.
(Other definitions for ethic that I've seen before include "System of rules of behaviour" , "type of behaviour" , "Moral code" , "Moral system" , "Ideology" .)