Comedian fairly includes afterthought at start of routine (8)
I believe the answer is:
quipster
'comedian' is the definition.
(thesaurus)
'fairly includes afterthought at start of routine' is the wordplay.
'fairly' becomes 'quite' (I've seen this before).
'includes' means one lot of letters goes inside another.
'afterthought' becomes 'ps' (as in a PS in a letter).
'at start of' suggests taking the first letters.
The initial letter of 'routine' is 'r'.
'quite' enclosing 'ps' is 'quipste'.
'quipste'+'r'='QUIPSTER'
(Other definitions for quipster that I've seen before include "wit" , "Maker of witty comments" , "wisecracker" .)