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In the beginning, long one assigned to cat? (8)

Ross

I believe the answer is:

lifeline

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

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

'beginning long one assigned to cat?' is the wordplay.
'beginning' indicates taking the first letters.
'one' becomes 'i' (Roman numeral).
'assigned to' means one lot of letters go next to another.
'cat?' becomes 'feline' (cat is a kind of feline).
The initial letter of 'long' is 'l'.
'l'+'i'+'feline'='LIFELINE'

'the' is the link.

Can you help me to learn more?

(Other definitions for lifeline that I've seen before include "Essential support (read by palmists?)" , "Vital safeguard" , "Essential means of support" , "Vital support?" , "Rope to haul person out of water - vital link" .)

I've seen this clue in The Telegraph.
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