Swelling bugle performance (5)
I believe the answer is:
bulge
'swelling' is the definition.
(I know that swelling can be written as bulge)
'bugle performance' is the wordplay.
'performance' indicates anagramming the letters.
'bugle' anagrammed gives 'BULGE'.
(Other definitions for bulge that I've seen before include "Hitler's offensive" , "Rounded swelling" , "Hump" , "Battle of the -----: last major German counter-offensive of WWII (1944)" , "Protrude" .)