Itinerant mounted police working across America (7)
I believe the answer is:
nomadic
'itinerant' is the definition.
(I've seen this before)
'mounted police working across america' is the wordplay.
'mounted' shows that the letters should be reversed in order (in a down clue, letters mounted up).
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'working' becomes 'on' (e.g. a machine is working or on).
'across' means one lot of letters goes inside another.
'america' becomes 'am'.
'cid'+'on'='cidon'
'cidon' placed around 'am' is 'cidamon'.
'cidamon' written backwards gives 'NOMADIC'.
(Other definitions for nomadic that I've seen before include "constantly moving" , "Unsettled" , "Migratory" , "Peripatetic" , "Itinerant" .)