Actual opening of marine area (5)
I believe the answer is:
realm
'area' is the definition.
(I know that realm is a type of area)
'actual opening of marine' is the wordplay.
'actual' becomes 'real' (synonyms).
'opening of' says to take the initial letters.
The first letter of 'marine' is 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "Lamer (anag.)" , "Kingdom or sphere" , "King's domain" , "Area of influence" , "field of study" .)