Jest involving me now crushed in disappointment (8)
I believe the answer is:
comedown
'disappointment' is the definition.
(comedown can mean a disappointment or anti-climax)
'jest involving me now crushed' is the wordplay.
'jest' becomes 'cod' (I can't justify this - if you can you should believe this answer much more).
'involving' means one lot of letters goes inside another.
'crushed' is an anagram indicator.
'cod' placed around 'me' is 'comed'.
'now' is an anagram of 'own'.
'comed'+'own'='COMEDOWN'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for comedown that I've seen before include "Anticlimax" , "Loss of position or status" , "lessening of its effect?" , "Loss of status, disappointment" , "Decline in status" .)