Sea side in an American state (6)
I believe the answer is:
'sea' is the definition.
(I've seen this before)
'side in an american state' is the wordplay.
'side' becomes 'r' (abbreviation for right).
'in' means one lot of letters goes inside another.
'an american state' becomes 'Maine' (Maine is an example).
'r' placed into 'maine' is 'MARINE'.
(Other definitions for marine that I've seen before include "jolly" , "Soldier associated with a navy" , "Relating to shipping or naval matters" , "Concerning the sea" , "Member of the forces" .)