Easy-going police department supporting port authority (6)
I believe the answer is:
placid
'easy' is the definition.
(thesaurus)
'police department supporting port authority' is the wordplay.
'police department' becomes 'CID' (abbreviation for Criminal Investigation Department).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'port authority' becomes 'pla' (Port of London Authority).
'cid' after 'pla' is 'PLACID'.
'going' acts as a link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for placid that I've seen before include "without ruffles" , "Peaceful and tranquil" , "Quiet and still" , "Calm of nature" , "Calm; equable" .)