Ring with master is for the Lord of the Underworld (6)
I believe the answer is:
osiris
'the lord of the underworld' is the definition.
I can't tell whether this definition defines the answer.
'ring with master is' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'with' is a charade indicator (letters next to each other).
'master' becomes 'sir' (a schoolmaster may be addressed as sir).
'o'+'sir'+'is'='OSIRIS'
'for' is the link.
Can you help me to learn more?
(Other definitions for osiris that I've seen before include "once highly esteemed by Egyptians" , "underworld boss" , "dead judge?" , "God, husband of Isis" , "one held to be divine" .)