Agreed: US city’s mostly in American state (8)
I believe the answer is:
oklahoma
'state' is the definition.
(Oklahoma is an example)
'agreed us city's mostly in american' is the wordplay.
'agreed' becomes 'ok' (eg when responding to an offer).
'us city' becomes 'LA' (Los Angeles).
'mostly' means to remove the last letter.
'in' becomes 'home' ('he's in' can mean 'he's home').
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'home' with its last letter taken away is 'hom'.
'ok'+'la'+'hom'+'a'='OKLAHOMA'
(Other definitions for oklahoma that I've seen before include "Young woman" , "A holm oak found in the state" , "American state and name of great musical" , "US state, joined in 1907" , "OK" .)