Group of jolly dimwits set up a happy scene (5)
I believe the answer is:
Here is my best explanation:
'a happy scene' is the definition.
'group of jolly dimwits set up' is the subsidiary indication.
'group of' indicates a hidden word (group of letters from the clue).
'set up' shows that the letters should be reversed in order.
'llydi' is hidden within 'jolly dimwits'.
'llydi' back-to-front is 'idyll'.
(Other definitions for idyll that I've seen before include "Utopian scene", "A charming scene", "Idealised prose", "Perfect situation", "very happy scene", "Pastoral composition", "Blissful, rustic, scene", "Pastoral work", "Blissful situation" and "Poem - charming scene".)