Inconclusive test on Italy's Adriatic peninsula (6)

'adriatic peninsula' is the definition.
(peninsula on the Adriatic Sea)

'inconclusive test on italy's' is the wordplay.
'inconclusive' means to remove the last letter.
'test' becomes 'trial' (synonyms).
'on' means one lot of letters go next to another.
'italy's' becomes 'i's'.
'trial' with its last letter removed is 'tria'.
'tria' after 'is' is 'ISTRIA'.