It's brilliant being big! (5)
I believe the answer is:
great
I believe this is a double definition.
'it's brilliant' is the first definition.
(thesaurus)
'big' is the second definition.
('great' can be similar in meaning to 'big')
'being' acts as a link.
(Other definitions for great that I've seen before include "'Goodness gracious -- -- -- --' (pop song)" , "Wonderful" , "Excellent (colloq.)" , "Waterbird" , "Huge or outstanding" .)