March day in Dorset ain't bad (13)
I believe the answer is:
demonstration
'march' is the definition.
(I've seen this before)
'day in dorset ain't bad' is the wordplay.
'day' becomes 'mon'.
'in' is an insertion indicator.
'bad' indicates an anagram.
'dorset'+'aint'='dorsetaint'
'dorsetaint' is an anagram of 'destration'.
'mon' placed into 'destration' is 'DEMONSTRATION'.
(Other definitions for demonstration that I've seen before include "Protest meeting" , "Display showing how something works" , "Public rally as protest" , "Protest march/meeting" , "Exhibition, protest" .)