Picturesque scene over in Caerphilly district (5)
I believe the answer is:
idyll
'picturesque scene' is the definition.
(I've seen this before)
'over in caerphilly district' is the wordplay.
'over' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'llydi' is hidden in the letters of 'caerphilly district'.
'llydi' written backwards gives 'IDYLL'.
(Other definitions for idyll that I've seen before include "Idealised prose" , "very happy situation" , "Poem or scene of pastoral simplicity" , "Scene of rural happiness" , "Happy period" .)