Party on sea or land (6)
I believe the answer is:
domain
'land' is the definition.
(I know that land can be written as domain)
'party on sea' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'on' means one lot of letters go next to another.
'sea' becomes 'main' (synonyms).
'do'+'main'='DOMAIN'
'or' acts as a link.
(Other definitions for domain that I've seen before include "Scope of knowledge - owned land" , "Territory over which rule or control is exercised" , "Realm or range of influence" , "Range of influence or field of thought" , "District or area - or range of influence" .)