Royal mistress supporting King produces ring (5)
I believe the answer is:
knell
'ring' is the definition.
(I know that knell is a more specific form of the action ring)
'royal mistress supporting king' is the wordplay.
'royal mistress' becomes 'nell' (I am not sure about the 'royal' bit.).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'king' becomes 'K' (chess abbreviation).
'nell' after 'k' is 'KNELL'.
'produces' is the link.
Can you help me to learn more?
(Other definitions for knell that I've seen before include "Solemn warning" , "Toll, ring" , "Solemnly proclaim death" , "(Of bell) ring solemnly" , "Sound of funeral bell" .)