Crossword Genius

French seen with old Mike, sometime Tube traveller? (4)

Ross

I believe the answer is:

ovum

I'm a little stuck... Click here to teach me more about this clue!

'tube traveller?' is the definition.
The definition and answer can be both body parts as well as being singular nouns.
Maybe you can see an association between them that I can't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for ovum that I've seen before include "A female egg-cell" , "Human egg cell" , "Reproductive cell, 'egg' in Latin" , "Female gamete" , "bit of life potentially" .)

I've seen this clue in The Times.
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