Getting a cabin starboard, say, is an inherited prerogative (10)
I believe the answer is:
birthright
'an inherited prerogative' is the definition.
'birthright' can be an answer for 'prerogative' (thesaurus). I am not sure about the 'an inherited' bit.
'getting a cabin starboard say is' is the wordplay.
'getting' says to put letters next to each other.
'a cabin' becomes 'irth' (I can't justify this - if you can you should believe this answer much more).
'starboard' becomes 'right' (synonyms).
'say' shows a homophone (sound like).
'is' becomes 'be'.
'be' is a homophone of 'b'.
'irth'+'right'='irthright'
'irthright' after 'b' is 'BIRTHRIGHT'.
Can you help me to learn more?
(Other definitions for birthright that I've seen before include "inheritance" , "Privilege" , "Something possessed from beginning of life" , "entitlement" , "Esau sold his" .)