It's widely known duck enclosures on island last given precedence (4,6)
I believe the answer is:
open secret
'island last given precedence' is the definition.
I can't judge whether this definition defines the answer.
'it's widely known duck enclosures' is the wordplay.
'it's widely known' becomes 'open' (I've seen this before).
'duck enclosures' becomes 'secret' (I can't explain this - if you can you should give a lot more credence to this answer).
'open'+'secret'='OPEN SECRET'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for open secret that I've seen before include "Centre pose (anag.)" , "common knowledge" , "Everyone knows it" , "Private matter actually known to many" , "Generally known matter" .)