Poor Kathleen stops totally heartless devil passing on ring (5,5)
I believe the answer is:
death knell
'passing on ring' is the definition.
(I've seen this before)
'poor kathleen stops totally heartless devil' is the wordplay.
'poor' indicates an anagram.
'stops' means one lot of letters goes inside another.
'totally heartless devil' becomes 'dl' (I am not sure about this - if you are sure you should believe this answer much more).
'kathleen' anagrammed gives 'eathknel'.
'eathknel' going within 'dl' is 'DEATH-KNELL'.
Can you help me to learn more?
(Other definitions for death knell that I've seen before include "Parting note" , "Passing on ring" , "passing on rings" , "This will end all the hopes" , "Bell rung when someone's popped their clogs" .)