Arrangement of sails aboard heavy eastern warship (7)
I believe the answer is:
frigate
'warship' is the definition.
(I know that frigate is a type of warship)
'arrangement of sails aboard heavy eastern' is the wordplay.
'arrangement of sails' becomes 'rig' (I am not sure about this - if you are sure you should believe this answer much more).
'aboard' means one lot of letters goes inside another.
'heavy' becomes 'fat' (similar in meaning).
'eastern' becomes 'e'.
'rig' put inside 'fat' is 'frigat'.
'frigat'+'e'='FRIGATE'
Can you help me to learn more?
(Other definitions for frigate that I've seen before include "Light warship" , "Warship; -- bird" , "Naval escort vessel" , "US warship" , "Warship - seabird" .)