In short, brilliant! (3)
I believe the answer is:
This clue could be a double definition.
'in' is the first definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'short brilliant' is the second definition.
(short for fabulous; 'short' asks for a short form)
Can you help me to learn more?
(Other definitions for fab that I've seen before include "Excellent (slang)" , "Marvellous (sl.)" .)