ring-fence
Concise Oxford English Dictionary © 2008 Oxford University Press:
ring fence
▶noun
- 1 a fence completely enclosing a piece of land.
- 2 an effective barrier.
- 1 enclose (land) with a ring fence.
- 2 Brit. guarantee that (funds for a particular purpose) will not be spent on anything else.
'ring-fence' also found in these Oxford entries:

