Butt of jokes is too gentlemanly, to some extent (6)
I believe the answer is:
stooge
'butt of jokes' is the definition.
(I've seen this before)
'is too gentlemanly to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'STOOGE' is hidden within 'is too gentlemanly'.
(Other definitions for stooge that I've seen before include "Comedian's sidekick" , "Performer who is butt of another's jokes" , "A comedian's stage butt or straight man" , "lackey" , "Comic's straight man" .)