Trim Boston style for musician (10)
I believe the answer is:
trombonist
'for musician' is the definition.
(trombonist is a kind of musician)
'trim boston style' is the wordplay.
'style' indicates anagramming the letters (style can mean to shape or design).
'trim'+'boston'='trimboston'
'trimboston' with letters rearranged gives 'TROMBONIST'.
(Other definitions for trombonist that I've seen before include "Ted Heath, perhaps" , "Brass player" .)