Cover up sick dun horse (8)
I believe the answer is:
enshroud
'cover up' is the definition.
(thesaurus)
'sick dun horse' is the wordplay.
'sick' indicates anagramming the letters.
'dun'+'horse'='dunhorse'
'dunhorse' is an anagram of 'ENSHROUD'.
(Other definitions for enshroud that I've seen before include "Cover with a garment (or mist?)" , "Hide - cloak" , "Completely envelop" , "Envelop and hide from view" .)