Stupid, happy band (10)
I believe the answer is:
blithering
'stupid' is the definition.
(I've seen this before)
'happy band' is the wordplay.
'happy' becomes 'blithe' (I've seen this before).
'band' becomes 'ring' (ring is a kind of band).
'blithe'+'ring'='BLITHERING'
(Other definitions for blithering that I've seen before include "Foolish" , "Stupid" , "talking foolishly" , "Contemptible" .)