Location upset Elizabeth. Most of Greek isle is restricted (3,6)
I believe the answer is:
top secret
'is restricted' is the definition.
(I've seen this before)
'location upset elizabeth most of greek isle' is the wordplay.
'location' becomes 'spot' (spot is a kind of location).
'upset' shows that the letters should be reversed in order.
'elizabeth' becomes 'e' (this could be a standard abbreviation which I don't know about).
'most of' means to remove the last letter.
'greek isle' becomes 'Crete'.
'spot' backwards is 'tops'.
'crete' with its final letter taken away is 'cret'.
'tops'+'e'+'cret'='TOP-SECRET'
Can you help me to learn more?
(Other definitions for top secret that I've seen before include "the knack of sleeping well?" , "label on some classified documents" , "Highly confidential" , "Erect post (anag)" , "Known to very few" .)