Fellow keyboard player loses one drummer (9)
I believe the answer is:
timpanist
'drummer' is the definition.
(timpanist is a kind of drummer)
'fellow keyboard player loses one' is the wordplay.
'fellow' becomes 'tim' (I have seen 'Little fellow' mean 'tim' so perhaps 'fellow' could also mean 'tim').
'keyboard player' becomes 'pianist' (a piano has a keyboard).
'loses' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'pianist' with 'i' taken out is 'panist'.
'tim'+'panist'='TIMPANIST'
(Other definitions for timpanist that I've seen before include "member of orchestra" , "I beat and roll" , "Drummer" .)