Composer in Swiss city with beer mug (9)
I believe the answer is:
bernstein
'composer' is the definition.
(Leonard Bernstein)
'swiss city with beer mug' is the wordplay.
'swiss city' becomes 'bern'.
'with' means one lot of letters go next to another.
'beer mug' becomes 'stein' ('stein' can be a synonym of 'beer mug').
'bern'+'stein'='BERNSTEIN'
'in' acts as a link.
(Other definitions for bernstein that I've seen before include "Noteworthy personality" , "Leonard --, American conductor" , "Mass producer" , "scoring ace" , "composer" .)