Eggs are very old when rejected (3)
I believe the answer is:
ova
'eggs' is the definition.
(term for egg cells)
'are very old when rejected' is the wordplay.
'are' becomes 'a' (short for 'are', historical unit of measurement).
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'when rejected' says the letters should be written in reverse.
'a'+'v'+'o'='avo'
'avo' backwards is 'OVA'.
(Other definitions for ova that I've seen before include "Prerequisite for procreation" , "Multiple Latin eggs" , "Release of women" , "Eggs; sounds like above" , "targets for male seed?" .)