Crossword Genius

Depth falling in say, flood level (4)

Ross

I believe the answer is:

tied

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

'level' is the definition.
(I know that level can be written as tied)

'depth falling in say flood' is the wordplay.
I cannot really understand how this works, but
'depth' could be 'd' (abbreviation in geometry) and 'd' is present in the answer.
The remaining letters 'tie' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for tied that I've seen before include "Drawn" , "Fastened with string" , "Bonded" , "Drew; bound" , "Made bow" .)

I've seen this clue in The Independent.
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