Enclosure is not simple (8)
I believe the answer is:
compound
This clue is a double definition.
'enclosure' is the first definition.
(compound is a kind of enclosure)
'not simple' is the second definition.
('compound' is the opposite to 'simple')
'is' acts as a link.
(Other definitions for compound that I've seen before include "Chemical substance; enclosed space" , "Whole formed by two or more elements" , "Composed of two or more separate elements" , "Fenced area; chemical substance" , "Enclosed area - not simple!" .)