Change it: MP Angela Rayner at one working on private bills (13,5)

I believe the answer is:

parliamentary agent

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'change it mp angela rayner at one working on private bills' is the definition.
I can't tell whether this defines the answer.

'mp angela rayner at one working on private bills' is the wordplay.
'one' becomes 'i' (Roman numeral).
'working' is an anagram indicator.
'on private bills' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'mp'+'angela'+'rayner'+'at'+'i'='mpangelaraynerati'
'mpangelaraynerati' with letters rearranged gives 'parliamentaryagen'.
'parliamentaryagen'+'t'='PARLIAMENTARY AGENT'

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I've seen this clue in The Guardian.

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