Sea water finally entering US state (6)
I believe the answer is:
marine
'sea' is the definition.
(I've seen this before)
'water finally entering us state' is the wordplay.
'water finally' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'entering' means one lot of letters goes inside another.
'us state' becomes 'maine' (Maine is a state of the USA).
'r' inserted into 'maine' is 'MARINE'.
Can you help me to learn more?
(Other definitions for marine that I've seen before include "Oceanic" , "Soldier-sailor" , "Relating to shipping or naval matters" , "Maritime - seagoing soldier" , "Soldier serving on land and sea" .)