Groovy balloon (5)
I believe the answer is:
swell
I believe this is a double definition.
'groovy' is the first definition.
(I know that groovy can be written as swell)
'balloon' is the second definition.
(I've seen this before)
(Other definitions for swell that I've seen before include "Excellent (slang)" , "Bulge; a nob" , "Movement of the surface of the sea" , "Grand" , "organ effect" .)