Retiring comedian, stressed about source of humour (9)
I believe the answer is:
withdrawn
'retiring' is the definition.
(thesaurus)
'comedian stressed about source of humour' is the wordplay.
'comedian' becomes 'wit' (a witty person).
'stressed' becomes 'drawn' (I've seen this before).
'about' says to put letters next to each other.
'source of' indicates taking the first letters (the start - or source - of the word).
The initial letter of 'humour' is 'h'.
'drawn' put after 'h' is 'hdrawn'.
'wit'+'hdrawn'='WITHDRAWN'
(Other definitions for withdrawn that I've seen before include "Retired" , "Shy, reserved" , "Shrinking" , "Private" , "Discontinued; shunning social contact" .)