Help witch periodically? It’s a long story (4)
I believe the answer is:
epic
'a long story' is the definition.
(eg an epic poem)
'help witch periodically?' is the wordplay.
'periodically?' means one should take alternating letters (letters must be selected 'every so often').
'help'+'witch'='helpwitch'
The alternate letters of 'helpwitch' are 'EPIC'.
'it's' acts as a link.
(Other definitions for epic that I've seen before include "Film or book on a grand scale" , "Long poem/story" , "imposing" , "Heroic narrative verse" , "long poem telling of heroic deeds" .)