No vice den is complete without this, you'll agree (4)
I believe the answer is:
cede
'agree' is the definition.
The definition and answer can be both to do with ownership as well as being verbs in their base form.
Perhaps there's an association between them I don't understand?
'no vice den is complete without this you'll' is the wordplay.
'CEDE' is hidden in amongst the letters.
However, I'm uncertain how this is indicated.
Can you help me to learn more?
(Other definitions for cede that I've seen before include "Allow" , "Deliver" , "Give away" , "Give up (power to another)" , "Give up one's rights" .)