Protective covering around slate unit (8)
I believe the answer is:
carapace
'protective covering' is the definition.
(thesaurus)
'around slate unit' is the wordplay.
'around' becomes 'ca' (circa).
'slate' becomes 'rap' (both can mean to criticise heavily).
'unit' becomes 'ace' (synonyms).
'ca'+'rap'+'ace'='CARAPACE'
(Other definitions for carapace that I've seen before include "Tortoise's upper shell" , "Hard outer covering of some animals" , "Hard shell of tortoise, crab etc" , "Hard thick shell, of a turtle say" , "Protective cover" .)