Ruby anniversary for union showing faithful heart? (8)

I believe the answer is:

fortieth

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

'ruby anniversary' is the definition.
(I've seen this before)

'for union showing faithful heart?' is the wordplay.
'union showing' becomes 'tie' (I can't justify this - if you can you should believe this answer much more).
'heart?' indicates the central letters (I've seen 'heart of' mean this).
The centre of 'faithful' is 'th'.
'for'+'tie'+'th'='FORTIETH'

Can you help me to learn more?

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

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

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