Island's milk suppliers are the tops (7)
I believe the answer is:
jerseys
'the tops' is the definition.
(I've seen this before)
'island's milk suppliers' is the wordplay.
'island' becomes 'jersey' (I've seen this before).
'milk suppliers' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'jersey'+'s'='JERSEYS'
'are' is the link.
Can you help me to learn more?
(Other definitions for jerseys that I've seen before include "Close-fitting pullovers" , "They provide milk" , "tops" .)