Ruby fastener found in river (8)

I believe the answer is:

fortieth

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

'ruby' is the definition.
(fortieth anniversaries are ruby)

'fastener found in river' is the wordplay.
'fastener' becomes 'tie' (I can't justify this - if you can you should give a lot more credence to this answer).
'found in' is an insertion indicator.
'river' becomes 'forth' (I've seen this before).
'tie' inserted within 'forth' is 'FORTIETH'.

Can you help me to learn more?

(Other definitions for fortieth that I've seen before include "Of weddings, ruby" , "Ruby anniversary" , "Ruby wedding?" , "Birthday" .)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.