King is greeting seer or sage (5)
I believe the answer is:
rishi
'seer or sage' is the definition.
(Hindu sage)
'king is greeting' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'greeting' becomes 'hi' (hi is a kind of greeting).
'r'+'is'+'hi'='RISHI'
(Other definitions for rishi that I've seen before include "Hindu sage or saint" , "Revered man" , "Spiritual leader" , "Poet" , "holy man" .)