Copy Rhode Island specialist in aid to motions (8)

I believe the answer is:

aperient

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'aid to motions' is the definition.
I can't judge whether this defines the answer.

'copy rhode island specialist' is the wordplay.
'copy' becomes 'ape' (ape can mean to copy or imitate).
'rhode island' becomes 'ri' (US postal abbreviation).
'specialist' becomes 'ent' (Ear, Nose and Throat specialist).
'ape'+'ri'+'ent'='APERIENT'

'in' is the link.

Can you help me to learn more?

(Other definitions for aperient that I've seen before include "Laxative medicine - repeat in prescription" , "drug?" , "Mildly laxative" .)

I've seen this clue in the Sydney Morning Herald.

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