Island school, one on a volcanic island (6)
I believe the answer is:
ischia
'island' is the definition.
(Italian island)
'island school one on a volcanic' is the wordplay.
'island' becomes 'i' (geographical abbreviation).
'school' becomes 'sch' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'on' means one lot of letters go next to another.
'volcanic' indicates an anagram (letters disrupted as if by a volcanic eruption).
'i' put after 'a' is 'ai'.
'ai' is an anagram of 'ia'.
'i'+'sch'+'ia'='ISCHIA'
(Other definitions for ischia that I've seen before include "Italian island" , "Volcanic island in the Bay of Naples" , "Parts of pelvis; isle near Naples" , "Island at the north end of the Bay of Naples" , "Insular offshore location" .)