Security device: ranch will install one in actuality (4-5)

I believe the answer is:

fire-alarm

'security device' is the definition.
(type of safety alarm)

'ranch will install one in actuality' is the wordplay.
'ranch' becomes 'farm' (I've seen this before).
'will install' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'in actuality' becomes 'real'.
'i'+'real'='ireal'
'farm' enclosing 'ireal' is 'FIRE-ALARM'.

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