Drink round south Mediterranean island mostly confidential (3,6)
I believe the answer is:
top secret
'confidential' is the definition.
(I've seen this before)
'drink round south mediterranean island mostly' is the wordplay.
'drink' becomes 'tope' (synonyms).
'round' means one lot of letters goes inside another.
'south' becomes 's' (abbreviation).
'mediterranean island' becomes 'Crete' (Greek island).
'mostly' means to remove the last letter (most but not all of the word).
'tope' going around 's' is 'topse'.
'crete' with its last letter removed is 'cret'.
'topse'+'cret'='TOP-SECRET'
(Other definitions for top secret that I've seen before include "visible to hardly anyone?" , "Like highly guarded information" , "As Bond mission" , "Highest classified (information)" , "Very hush-hush" .)