Ring of King Charles II's mistress (5)
I believe the answer is:
knell
'ring' is the definition.
(I know that ring can be written as knell)
'king charles ii's mistress' is the wordplay.
'king' becomes 'K' (chess abbreviation).
'charles ii's mistress' becomes 'nell' (I've seen this in another clue).
'k'+'nell'='KNELL'
'of' is the link.
(Other definitions for knell that I've seen before include "Sound of funeral bell" , "Funeral toll" , "Toll, ring" , "it precipitates death" , "(Of bell) ring solemnly" .)