Organist initially changes organs (7)
I believe the answer is:
ovaries
'organs' is the definition.
(ovary is a kind of organ)
'organist initially changes' is the wordplay.
'initially' suggests taking the first letters.
'changes' becomes 'varies' ('vary' can be a synonym of 'change').
The initial letter of 'organist' is 'o'.
'o'+'varies'='OVARIES'
(Other definitions for ovaries that I've seen before include "egg production unit" , "Egg-producing organs in female animals" .)