Greeting of peace to welcome king is a custom (6)
I believe the answer is:
praxis
'a custom' is the definition.
(thesaurus)
'greeting of peace to welcome king' is the wordplay.
'greeting' becomes 'pax' (pax is a kind of greeting).
'of peace' becomes 'is' (I can't explain this - if you can you should believe this answer much more).
'to welcome' indicates putting letters inside (in sense of 'inviting in').
'king' becomes 'R' (abbreviation for rex, king in Latin).
'pax'+'is'='paxis'
'paxis' going around 'r' is 'PRAXIS'.
'is' is the link.
Can you help me to learn more?
(Other definitions for praxis that I've seen before include "Practice, as distinct from theory, of an art or science" , "Working out of theory" , "Application of theory" , "accepted custom" .)