Declares a piece of poetry endless (5)
I believe the answer is:
avers
'declares' is the definition.
(averring is a kind of declaring)
'a piece of poetry endless' is the wordplay.
'piece of poetry' becomes 'verse' (I've seen this before).
'endless' means to remove the last letter.
'verse' with its final letter removed is 'vers'.
'a'+'vers'='AVERS'
(Other definitions for avers that I've seen before include "Swears" , "Alleges, asserts" , "States firmly - really raves" , "States to be true" , "States positively" .)