Someone from Belgrade is happy inside - that's fantastic (6)
I believe the answer is:
superb
'fantastic' is the definition.
(I've seen this before)
'someone from belgrade is happy inside' is the wordplay.
'someone from belgrade' becomes 'Serb' (Belgrade is the capital of Serbia).
'is happy' becomes 'up' (I've seen this in other clues).
'inside' indicates putting letters inside.
'serb' placed around 'up' is 'SUPERB'.
'that's' acts as a link.
(Other definitions for superb that I've seen before include "Outstandingly excellent" , "Highly impressive" , "Splendid, marvellous" , "Splendid, brilliant" , "Excellent, magnificent" .)