To start with, Electric Picnic is cancelled - it's a long story (4)
I believe the answer is:
epic
'a long story' is the definition.
(eg an epic poem)
'to start with electric picnic is cancelled' is the wordplay.
'to start with' indicates taking the first letters.
The initial letters of 'electric picnic is cancelled' is 'EPIC'.
'it's' is the link.
(Other definitions for epic that I've seen before include "On a grand scale, like heroic poem" , "(Of) immense (proportions)" , "Long poem etc" , "Guardian (body, beast)" , "of impressive proportions?" .)