Litter can upset first musician (12)
I believe the answer is:
clarinettist
'musician' is the definition.
(I know that clarinettist is a type of musician)
'litter can upset first' is the wordplay.
'upset' indicates anagramming the letters.
'first' becomes 'ist' (ist resembles 1st).
'litter'+'can'='littercan'
'littercan' with letters rearranged gives 'clarinett'.
'clarinett'+'ist'='CLARINETTIST'
(Other definitions for clarinettist that I've seen before include "Musician, like Benny Goodman, say" , "Cats in litter (anag) -- woodwind player" .)