Doctor sure moon is huge (8)
I believe the answer is:
enormous
'huge' is the definition.
('enormous' can be a synonym of 'huge')
'doctor sure moon' is the wordplay.
'doctor' indicates an anagram.
'sure'+'moon'='suremoon'
'suremoon' anagrammed gives 'ENORMOUS'.
'is' is the link.
(Other definitions for enormous that I've seen before include "Huge, tremendous" , "Tremendous, gigantic" , "Extremely large" , "Immense" , "Vast" .)