One knocks off arrangement for clarinets (9)
I believe the answer is:
larcenist
'one knocks off' is the definition.
'arrangement for clarinets' is the wordplay.
'arrangement for' indicates an anagram (I've seen this in other clues).
'clarinets' with letters rearranged gives 'LARCENIST'.
(Other definitions for larcenist that I've seen before include "One steals clarinets, oddly" , "Thief" , "Dishonest person" , "One criminal" , "Person who takes" .)