Old changes to organs (7)
I believe the answer is:
ovaries
'to organs' is the definition.
(ovary is a kind of organ)
'old changes' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'changes' becomes 'varies' ('vary' can be a synonym of 'change').
'o'+'varies'='OVARIES'
(Other definitions for ovaries that I've seen before include "Egg-producing organs in female animals" , "egg production unit" .)