Curtain level in hole (8)
I believe the answer is:
Here is my best explanation:
'curtain level in hole' is the definition.
'portiere' can be an answer for 'curtain' (I've seen this before). I am not sure about the 'level in hole' bit.
'curtain level in hole' is the subsidiary indication.
'curtain level' becomes 'tier' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'hole' becomes 'pore' (pore is a kind of hole).
'tier' inserted within 'pore' is 'portiere'.
(Other definitions for portiere that I've seen before include "Curtain" and "curtain over door".)