Ranch dressing one genuine reason to leave restaurant in a hurry (4,5)

I believe the answer is:

fire alarm

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'reason to leave restaurant in a hurry' is the definition.
I can't tell whether this definition defines the answer.

'ranch dressing one genuine' is the wordplay.
'ranch' becomes 'farm' (I've seen this before).
'dressing' means one lot of letters goes inside another (inserted letters are dressed or clad in others).
'one' becomes 'i' (Roman numeral).
'genuine' becomes 'real' (similar in meaning).
'i'+'real'='ireal'
'farm' going around 'ireal' is 'FIRE-ALARM'.

Can you help me to learn more?

(Other definitions for fire alarm that I've seen before include "Safety equipment" , "Warning of conflagration" , "Warning to evacuate" , "Warner" , "It warns of smoke and flames" .)

I've seen this clue in The Independent.

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