Group of jolly dimwits set up a happy scene (5)
I believe the answer is:
idyll
'a happy scene' is the definition.
'group of jolly dimwits set up' is the wordplay.
'group of' indicates a hidden word (group of letters from the clue).
'set up' is a reversal indicator.
'llydi' can be found hidden inside 'jolly dimwits'.
'llydi' in reverse letter order is 'IDYLL'.
(Other definitions for idyll that I've seen before include "Poem - charming scene" , "Pastoral work" , "piece of poetry" , "Short pastoral poem" , "Pictorial poem" .)