Finally gone — a clever, entertaining setter, undoubtedly! (6)
I believe the answer is:
easily
'undoubtedly' is the definition.
(thesaurus)
'finally gone a clever entertaining setter' is the wordplay.
'finally' indicates one should take the final letters.
'clever' becomes 'sly' (I've seen this in another clue).
'entertaining' is an insertion indicator.
'setter' becomes 'I' (the person that set the crossword).
The final letter of 'gone' is 'e'.
'sly' placed around 'i' is 'sily'.
'e'+'a'+'sily'='EASILY'
(Other definitions for easily that I've seen before include "With little difficulty" , "Without difficulty or effort" , "Without much difficulty" , "with little effort" , "without a hitch?" .)