Player is among leaders of our best orchestras today (6)
I believe the answer is:
oboist
'player' is the definition.
(oboe player)
'is among leaders of our best orchestras today' is the wordplay.
'among' means one lot of letters goes inside another.
'leaders of' indicates taking the first letters.
The first letters of 'our best orchestras today' is 'obot'.
'is' put into 'obot' is 'OBOIST'.
(Other definitions for oboist that I've seen before include "Do I boost the musician?" , "Wind player" , "Instrumentalist" , "Musician who plays woodwind of treble pitch" , "orchestra member" .)