Calm father's left inside with police (6)
I believe the answer is:
placid
'calm' is the definition.
(synonyms)
'father's left inside with police' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'left' becomes 'l' (common abbreviation).
'inside' indicates putting letters inside.
'with' means one lot of letters go next to another.
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'pa' placed around 'l' is 'pla'.
'pla'+'cid'='PLACID'
(Other definitions for placid that I've seen before include "Peaceful - composed" , "Peaceable and not easily disturbed" , "Quiet and still" , "mild" , "Calm; equable" .)