Singer in bar, one over in centre of Montreux (12)
I believe the answer is:
countertenor
'singer' is the definition.
(countertenor is a kind of singer)
'bar one over in centre of montreux' is the subsidiary indication.
'bar' becomes 'counter' (I've seen this in other clues**).
'over' indicates anagramming the letters.
'in' is an insertion indicator.
'centre of' means to look at the middle letters.
The middle of 'montreux' is 'tr'.
'one' with letters rearranged gives 'eno'.
'eno' put within 'tr' is 'tenor'.
'counter'+'tenor'='countertenor'
'in' is the link.
(Other definitions for countertenor that I've seen before include "he'll sing" , "Singer" , "Highest male singing voice" , "being highly vocal" , "Adult male with an alto voice" .)