Jewel used in ring worn by monarch (5)
I believe the answer is:
pearl
'jewel' is the definition.
(I know that pearl is a type of jewel)
'ring worn by monarch' is the wordplay.
'ring' becomes 'peal' (pealing is a kind of ringing).
'worn by' is an insertion indicator (outside letters are worn by those on the inside).
'monarch' becomes 'R' (abbreviation for rex, king in Latin).
'peal' enclosing 'r' is 'PEARL'.
'used in' acts as a link.
(Other definitions for pearl that I've seen before include "one regularly in stitches" , "Lustrous structure inside an oyster-shell" , "White - jewel" , "coming out of shell?" , "Drop" .)