Quiet, left a criminal investigation division calm (6)
I believe the answer is:
placid
'calm' is the definition.
(synonyms)
'quiet left a criminal investigation division' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'left' becomes 'l' (common abbreviation).
'criminal investigation division' becomes 'cid' (this could be a standard abbreviation of which I'm unaware).
'p'+'l'+'a'+'cid'='PLACID'
Can you help me to learn more?
(Other definitions for placid that I've seen before include "Serene - unruffled" , "Peaceful - composed" , "not easily angered" , "Calm and tranquil" , "Tranquil, unruffled" .)