Calamitous experience carrying disease endlessly spread (9)
I believe the answer is:
marmalade
'spread' is the definition.
(I've seen this before)
'calamitous experience carrying disease endlessly' is the wordplay.
'calamitous experience' becomes 'mare' (I can't explain this - if you can you should give a lot more credence to this answer).
'carrying' indicates putting letters inside.
'disease' becomes 'malady' (malady is a kind of health problem).
'endlessly' means to remove the last letter.
'malady' with its final letter taken off is 'malad'.
'mare' enclosing 'malad' is 'MARMALADE'.
Can you help me to learn more?
(Other definitions for marmalade that I've seen before include "Preserve made with citrus fruits" , "Kind of cat" , "Eleven's [PRESERVE]" , "group?" , "Spread" .)