Nothing is mutable in these organs (7)
I believe the answer is:
ovaries
'these organs' is the definition.
(ovary is a kind of organ)
'nothing is mutable' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'is mutable' becomes 'varies' (something which varies is mutable or changing).
'o'+'varies'='OVARIES'
'in' is the link.
(Other definitions for ovaries that I've seen before include "Egg-producing organs in female animals" , "egg production unit" .)