Crossword Genius

Made fast when so married (4,3,4)

Ross

I believe the answer is:

tied the knot

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

'made fast when' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'married' is the wordplay.
I cannot quite see how this works, but
'married' could be 'tied' ('tie' can be a synonym of 'marry') and 'tied' is located in the answer.
This explanation may well be incorrect...

'so' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.
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