Donor is sick inside (7)
I believe the answer is:
indoors
'inside' is the definition.
(synonyms)
'donor is sick' is the wordplay.
'sick' indicates an anagram.
'donor'+'is'='donoris'
'donoris' anagrammed gives 'INDOORS'.
(Other definitions for indoors that I've seen before include "Within a building" , "not the place for outsiders?" , "Within house" , "under cover" , "Having returned to pavilion" .)