Partner developing sea-coast around island (9)
I believe the answer is:
associate
'partner' is the definition.
(thesaurus)
'developing sea-coast around island' is the wordplay.
'developing' indicates anagramming the letters.
'around' means one lot of letters goes inside another.
'island' becomes 'i'.
'sea'+'coast'='seacoast'
'seacoast' is an anagram of 'assocate'.
'assocate' going around 'i' is 'ASSOCIATE'.
(Other definitions for associate that I've seen before include "Link" , "Partner, colleague" , "Join" , "Get together with" , "Make a logical or causal connection" .)