Is participating in march — heaven! (8)
I believe the answer is:
paradise
'heaven' is the definition.
(paradise is a kind of heaven)
'is participating in march' is the wordplay.
'participating in' is an insertion indicator.
'march' becomes 'parade' (parading is a kind of marching).
'is' put into 'parade' is 'PARADISE'.
(Other definitions for paradise that I've seen before include "limbo" , "Felicity" , "Idyllic place" , "Heaven, nirvana, Valhalla etc" , "Scene of bliss" .)