Soldier-king in US state (6)
I believe the answer is:
marine
'soldier' is the definition.
(I know that marine is a type of soldier)
'king in us state' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'in' indicates putting letters inside.
'us state' becomes 'maine' (Maine is a state of the USA).
'r' placed within 'maine' is 'MARINE'.
(Other definitions for marine that I've seen before include "jolly" , "seaman" , "Relating to shipping or naval matters" , "'Nautical, maritime (6)'" , "Maritime - seagoing soldier" .)