Wonderful, but all in the mind (9)
I believe the answer is:
fantastic
I believe this clue is a double definition.
'wonderful' is the first definition.
('fantastic' can be a synonym of 'wonderful')
'all in the mind' is the second definition.
(thesaurus)
'but' is the link.
(Other definitions for fantastic that I've seen before include "Terrific" , "Splendid, excellent" , "Weird" , "Wicked" , "Dandy, maybe" .)