Ring Eleanor at the King's Head (5)
I believe the answer is:
knell
'ring' is the definition.
(I know that knell is a more specific form of ring)
'eleanor at the king's head' is the wordplay.
'eleanor' becomes 'nell' (nickname for Eleanor).
'at' means one lot of letters go next to another (I've seen this in other clues).
'head' suggests taking the first letters.
The first letter of 'kings' is 'k'.
'nell' put after 'k' is 'KNELL'.
(Other definitions for knell that I've seen before include "Solemnly proclaim death" , "Solemn warning" , "Toll (bell)" , "Awesome ring" , "Toll, ring" .)