Operative as a member of a posse (7)

I believe the answer is:

inforce

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'operative' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'as a member of a posse' is the wordplay.
'as' becomes 'in' (I've seen this in other clues).
'a member of a posse' becomes 'force' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in'+'force'='INFORCE'

Can you help me to learn more?

I've seen this clue in The Irish News.

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