Peaceful period badly interrupted by discontented donkey (5)
I believe the answer is:
idyll
'peaceful period' is the definition.
(idyll can mean a peaceful or perfect period)
'badly interrupted by discontented donkey' is the wordplay.
'badly' becomes 'i'll' (eg in ill-advised).
'interrupted by' means one lot of letters goes inside another.
'discontented' suggests removing the centre (the word's contents are removed).
'donkey' with its centre taken out is 'dy'.
'ill' placed around 'dy' is 'IDYLL'.
(Other definitions for idyll that I've seen before include "Blissful situation" , "Bucolic state" , "Short pastoral poem" , "Romantic poem or scene" , "Simple story" .)