An island chief overthrown for drinking (7)
I believe the answer is:
potable
'an island' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'chief overthrown for drinking' is the wordplay.
'chief' becomes 'top' (synonyms).
'overthrown' says the letters should be written in reverse.
'for drinking' becomes 'able' (I can't explain this - if you can you should believe this answer much more).
'top' back-to-front is 'pot'.
'pot'+'able'='POTABLE'
Can you help me to learn more?
(Other definitions for potable that I've seen before include "may be drunk" , "Able to be drunk" , "Drinkable" , "Non-toxic" , "fit to ingest" .)