Party approaching Sea World (6)
I believe the answer is:
domain
'world' is the definition.
('domain' can be a synonym of 'world')
'party approaching sea' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'approaching' is a charade indicator (letters next to each other).
'sea' becomes 'main' ('main' can be a synonym of 'sea').
'do'+'main'='DOMAIN'
(Other definitions for domain that I've seen before include "Range of influence or field of activity" , "Range of influence or field of thought" , "Sphere or territory" , "Sphere of activity" , "Territory over which someone reigns" .)