A sea group reassembled in Athenian court (9)
I believe the answer is:
areopagus
'athenian court' is the definition.
(I've seen this before)
'a sea group reassembled' is the wordplay.
'reassembled' indicates anagramming the letters.
'sea'+'group'='seagroup'
'seagroup' is an anagram of 'reopagus'.
'a'+'reopagus'='AREOPAGUS'
'in' acts as a link.
(Other definitions for areopagus that I've seen before include "Athenian court" , "Ancient Greek court; ago, as pure (anag.)" .)