Irregular poem could be listless (4)
I believe the answer is:
mope
'be listless' is the definition.
(to mope is to be listless or despondent)
'irregular poem' is the wordplay.
'irregular' is an anagram indicator.
'poem' with letters rearranged gives 'MOPE'.
'could' is the link.
(Other definitions for mope that I've seen before include "Sulk; mooch" , "Moon around" , "listless person" , "Wander round gloomily" , "Hang around being miserable" .)