Player of bassoon, or instrument similarly thin, first of all (6)
I believe the answer is:
oboist
'player' is the definition.
(oboe player)
'of bassoon or instrument similarly thin first of all' is the wordplay.
'of' becomes 'o''.
'first of all' says to take the initial letters.
The initial letters of 'bassoon or instrument similarly thin' is 'boist'.
'o'+'boist'='OBOIST'
(Other definitions for oboist that I've seen before include "Instrumentalist" , "Player of woodwind instrument of treble pitch" , "Musician who plays woodwind of treble pitch" , "Wind player" , "Do I boost the musician?" .)