Female travelling to their ruby wedding anniversary (8)

I believe the answer is:

fortieth

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

'ruby wedding anniversary' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'female travelling to their' is the wordplay.
'female' becomes 'f' (abbreviation).
'travelling' indicates anagramming the letters.
'to'+'their'='totheir'
'totheir' anagrammed gives 'ortieth'.
'f'+'ortieth'='FORTIETH'

Can you help me to learn more?

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

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.