Crossword Genius

Top desire to have son promoted (6)

Ross

I believe the answer is:

tshirt

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

'top' is the definition.
(I know that Tshirt is a type of shirt)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

I've seen this clue in The Irish Times.
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