Film old relative during journey on horseback (3,6)

I believe the answer is:

rio grande

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'film' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'old relative during journey on horseback' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'relative' becomes 'gran' (I've seen this in other clues).
'during' indicates putting letters inside.
'journey on horseback' becomes 'ride' (I've seen this before).
'o'+'gran'='ogran'
'ogran' put into 'ride' is 'RIO GRANDE'.

Can you help me to learn more?

(Other definitions for rio grande that I've seen before include "US-Mexico border river" , "Long river" , "American flower" , "Waterway rising in Colorado" , "running water" .)

I've seen this clue in The Times.

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