Mixture — it's enclosed (8)
I believe the answer is:
compound
I believe this clue is a double definition.
'mixture' is the first definition.
(thesaurus)
'it's enclosed' is the second definition.
(enclosed area)
(Other definitions for compound that I've seen before include "POW camp" , "In this fracture, bone has pierced the skin" , "Combination of elements - make worse - fenced-in area" , "Chemical substance; enclosed space" , "Fenced area; chemical substance" .)