Musician's ear, say, is tested, initially (8)
I believe the answer is:
organist
'musician's' is the definition.
(organist is a kind of musician)
'ear say is tested initially' is the wordplay.
'ear say' becomes 'organ' (ear is a kind of organ).
'initially' suggests taking the first letters.
The initial letter of 'tested' is 't'.
'organ'+'is'+'t'='ORGANIST'
(Other definitions for organist that I've seen before include "Cathedral musician" , "Church instr. player" , "Accompanist, keyboard player" , "Roasting (anag)" , "One plays in church usually" .)