Admit altering order for small marine arm (5)
I believe the answer is:
inlet
'small marine arm' is the definition.
(an inlet is a small arm of a body of water)
I cannot understand how the rest of the clue works.
(Other definitions for inlet that I've seen before include "Gulf" , "Bay, cove" , "Small area of sea" , "Opening through which liquid can flow" , "Small opening of coastline" .)