Clarinet playing Debussy’s last piece naturally (9)
I believe the answer is:
certainly
'naturally' is the definition.
(thesaurus)
'clarinet playing debussy's last' is the wordplay.
'playing' is an anagram indicator.
'last' says to take the final letters.
The last letter of 'debussy' is 'y'.
'clarinet' anagrammed gives 'certainl'.
'certainl'+'y'='CERTAINLY'
'piece' is the link.
(Other definitions for certainly that I've seen before include "Of course" , "Thirty seven [37D=WITHOUT FAIL]" , "Definitely, surely" , "By all means" , "Assuredly" .)