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