Diplomat’s covetousness involving ring (5)
I believe the answer is:
envoy
'diplomat's' is the definition.
(envoy is a kind of diplomat)
'covetousness involving ring' is the wordplay.
'covetousness' becomes 'envy' (to covet something is to feel envy for it).
'involving' means one lot of letters goes inside another.
'ring' becomes 'O' (letter resembling a ring).
'envy' enclosing 'o' is 'ENVOY'.
(Other definitions for envoy that I've seen before include "Diplomatic messenger" , "Ambassador" , "final words in writing" , "Plenipotentiary" , "One sent on a mission" .)