System of rules and suchlike hosts welcome (5)
I believe the answer is:
ethic
'system of rules' is the definition.
(ethic is a kind of system of rules)
'and suchlike hosts welcome' is the wordplay.
'and suchlike' becomes 'etc' (et cetera).
'hosts' indicates putting letters inside (inserted letters are welcomed or hosted).
'welcome' becomes 'hi' (both are greetings).
'etc' going around 'hi' is 'ETHIC'.
(Other definitions for ethic that I've seen before include "Moral rule" , "System of principles" , "Moral value" , "Set of moral values" , "Moral principle" .)