Crossword Genius

What made Daddy so tall? (4,4)

Ross

I believe the answer is:

long legs

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

'what made daddy' is the definition.
I know nothing about this answer so I can't tell whether this works.

'tall?' is the wordplay.
I cannot really understand how this works, but
'tall?' could be 'long' (I've seen this before) and 'long' is found in the answer.
The remaining letters 'legs' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'so' acts as a link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.
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