Long, possibly endless piece? (4)
I believe the answer is:
epic
'long' is the definition.
(as in an epic story)
'possibly endless piece?' is the wordplay.
'possibly' indicates an anagram.
'endless' means to remove the last letter.
'piece' with its final letter taken away is 'piec'.
'piec' with letters rearranged gives 'EPIC'.
(Other definitions for epic that I've seen before include "Guardian (body, beast)" , "Film or book on a grand scale" , "Long story telling of a hero's deeds" , "Impressively great" , "Long poem etc" .)