This island is known to be conservative, perhaps (4)
I believe the answer is:
tory
'this island is known' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'to be conservative perhaps' is the wordplay.
'be conservative perhaps' becomes 'ry' (I can't justify this - if you can you should give a lot more credence to this answer).
'to'+'ry'='TORY'
Can you help me to learn more?
(Other definitions for tory that I've seen before include "type on right? Yes" , "Politician; tyro (anag.)" , "English Conservative politician or Donegal island" , "blue" , "A British Conservative" .)