Idiot greeting King Edward’s tailor (8)
I believe the answer is:
clothier
'tailor' is the definition.
(I've seen this before)
'idiot greeting king edward's' is the wordplay.
'idiot' becomes 'clot' (I've seen this before).
'greeting' becomes 'hi' (hi is a kind of greeting).
'king edward' becomes 'er' (abbreviation for Eduardus Rex).
'clot'+'hi'+'er'='CLOTHIER'
(Other definitions for clothier that I've seen before include "Shopkeeper" , "Person who sells garments" , "who has particular habits?" , "Merchant selling garments" , "Merchant who sells men's suits etc" .)