Return of police force? A day is not settled (7)
I believe the answer is:
nomadic
'is not settled' is the definition.
(I've seen this before)
'return of police force? a day' is the wordplay.
'return of' says the letters should be written in reverse.
'police force?' becomes 'CID' (Criminal Investigation Department).
'day' becomes 'mon' (abbreviation for Monday).
'cid'+'a'+'mon'='cidamon'
'cidamon' written backwards gives 'NOMADIC'.
(Other definitions for nomadic that I've seen before include "migrant" , "Unsettled" , "It's just like Rover" , "Migratory" , "Perhaps unsettling" .)