Colleague developed sea-coast around island (9)
I believe the answer is:
associate
'colleague' is the definition.
(thesaurus)
'developed sea-coast around island' is the wordplay.
'developed' indicates anagramming the letters.
'around' indicates putting letters inside.
'island' becomes 'i'.
'sea'+'coast'='seacoast'
'seacoast' is an anagram of 'assocate'.
'assocate' placed around 'i' is 'ASSOCIATE'.
(Other definitions for associate that I've seen before include "Person one works or spends time with" , "Hobnob" , "Partner, colleague" , "Make a logical or causal connection" , "Connect; partner" .)