Scornful humorist disheartened swimmer... (9)
I believe the answer is:
withering
'scornful' is the definition.
(thesaurus)
'humorist disheartened swimmer' is the wordplay.
'humorist' becomes 'wit' (wit is a kind of humorist).
'disheartened' says to hollow out the word (remove centre letters).
'swimmer' becomes 'herring' (herring is a type of fish).
'herring' with its centre removed is 'hering'.
'wit'+'hering'='WITHERING'
(Other definitions for withering that I've seen before include "Devastating" , "Supercilious" .)