Port authority police, still unmoved (6)

I believe the answer is:

placid

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'unmoved' is the definition.
(thesaurus)

'port authority police still' is the wordplay.
'port authority' becomes 'pla' (Port of London Authority).
'police still' becomes 'cid' (abbreviation for Criminal Investigation Department. I am not sure about the 'still' bit.).
'pla'+'cid'='PLACID'

Can you help me to learn more?

(Other definitions for placid that I've seen before include "without ruffles" , "Calm; equable" , "Collected" , "Calm and not easily upset" , "Calm; Lake" .)

I've seen this clue in the Evening Standard.

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