Reproductive cells (3)
'reproductive cells' is the definition.
(ovum is a kind of reproductive cell)
This is the entire clue.
(Other definitions for ova that I've seen before include "Multiple Latin eggs" , "Likely end for Russian Fed cup players" , "Female reproductive cells" , "Eggs; sounds like above" , "Release of women" .)
(Another definition for spores that I've seen is " Tiny reproductive cells as of fungi or ferns".)